A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs <image> and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

Input

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

Output

Print single integer: the minimum cost of tickets needed to visit all schools.

Examples

Input

2


Output

0


Input

10


Output

4

Note

In the first example we can buy a ticket between the schools that costs <image>.
//package cp;
import java.io.*;
import java.lang.reflect.Array;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.*;

public class Main{

    static long mod=1000000007;
    static int tennine=1000000000;
    static int dp[][];
    static ArrayList<ArrayList<pair>> listList;
    static ArrayList<Long> lolist;
    static ArrayList<pair> plist;
    static ArrayList<Integer> inList;
    static int n;
    static int low[];
    static int high[];
    static int dfsOrder[];
    static int count;
    static long segArr[];
    static long weightArr[];
    static long w[];
    static int map[];
    static long ans[];
    static long weightPre[];
    static char arr[];
    static int cost[][];
    static boolean incoming[];
    static boolean outgoing[];
    public static void main(String[] args) throws IOException {
    	SReader sc=new SReader();
    	//Reader sc=new Reader();
    	int n=sc.ni();
    	if(n==1)
    		System.out.println(0);
    	else if(n%2==0)
    		System.out.println((n/2)-1);
    	else
    		System.out.println((n/2));
    }
    public static boolean can(long arr[])
    {
    	for(int i=2;i<n;i++)
    	{
    		if(arr[i-1]+arr[i-2]>arr[i])
    			return true;
    	}
    	return false;
    }
    public static boolean dfs(int node,int target,boolean visited[])
    {
    	if(node==target)
    		return true;
    	if(visited[node])
    		return false;
    	visited[node]=true;
    	boolean poss=false;
    	if(!outgoing[node])
    	{
    		return false;
    	}
    	if(node-1!=-1&&incoming[node-1])
    	{
    		poss=poss||dfs(node-1,target,visited);
    	}
    	if(node+1!=n&&incoming[node+1])
    	{
    		poss=poss||dfs(node+1,target,visited);
    	}
    	return poss;
    }
    public static void printMap(Map mp) {
        Iterator it = mp.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pair = (Map.Entry) it.next();
            System.out.println(pair.getKey() + " = " + pair.getValue());
            it.remove(); // avoids a ConcurrentModificationException
        }
    }
    static long gcd(long a, long b)
    {
        if (a == 0)
            return b;
        return gcd(b % a, a);
    }

    // method to return LCM of two numbers
    static long lcm(long a, long b)
    {
        return (a*b)/gcd(a, b);
    }

    static void bin(int n)
    {
        if (n > 1)
            bin(n/2);
        System.out.print(n % 2);
    }
    static long power(long x, long y, long p)
    {
        long res = 1;
        x = x % p;

        while (y > 0)
        {
            if((y & 1)==1)
                res = (res * x) % p;
            y = y >> 1;
            x = (x * x) % p;
        }
        return res;
    }
    static int next(int a[],int start)
    {
        for(int i=start+1;i<a.length;i++)
        {
            if(a[i]!=-1) {
                return i;
            }
        }
        return 0;
    }
    static long g(long a, long b) {
        if (b == 0)
            return a;
        return g(b, a % b);
    }

    public static boolean equal(ArrayList<Integer> a, ArrayList<Integer> b) {
        if (a.size() != b.size())
            return false;
        for (int i = 0; i < a.size(); i++) {
            if (a.get(i) != b.get(i))
                return false;
        }
        return true;
    }

    static int bins_first_pos(ArrayList<Integer> list, int ele) {
        int pos1 = Collections.binarySearch(list, ele);
        if (pos1 > -1) {
            int pos2 = Collections.binarySearch(list, ele - 1);
            if (pos2 < 0) {
                pos2 = Math.abs(pos2) - 1;
                return pos2;
            } else {
                for (int i = pos2; i <= pos1; i++) {
                    if (ele == list.get(i))
                        return i;
                }
            }
        } else
            return pos1;
        return 0;
    }


    static class bfs_ele {
        ArrayList<Integer> list;
        int left_sum;
        ArrayList<Integer> left_ele;

        bfs_ele() {
            list = new ArrayList<>();
            left_ele = new ArrayList<>();
            left_sum = 0;
        }

        void init_sum(int a) {
            left_sum = a;
        }
    }

    static class trie {
        long a;
        long b;
        long c;

        trie(long a, long b, long c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }

        trie() {
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (!(o instanceof trie)) return false;

            trie trie = (trie) o;

            if (a != trie.a) return false;
            if (b != trie.b) return false;
            return c == trie.c;
        }

        @Override
        public int hashCode() {
            int result = (int) (a ^ (a >>> 32));
            result = 31 * result + (int) (b ^ (b >>> 32));
            result = 31 * result + (int) (c ^ (c >>> 32));
            return result;
        }
    }

    static class pair {
        long a;
        long b;

        pair(long a, long b) {
            this.a = a;
            this.b = b;
        }
        public String toString()
        {
            return("a: "+a+" b: "+b);
        }
        pair() {
        }
        public pair getReversePair()
        {
            return new pair(b,a);
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (!(o instanceof pair)) return false;

            pair pair = (pair) o;

            if (a != pair.a) return false;
            return b == pair.b;
        }

        @Override
        public int hashCode() {
            int result = (int) (a ^ (a >>> 32));
            result = 31 * result + (int) (b ^ (b >>> 32));
            return result;
        }
    }

    static class TrieComparator implements Comparator<trie> {
        public int compare(trie c1, trie c2) {
            return Long.compare(c1.c, c2.c);
        }
    }

    static class PairComparatorpos implements Comparator<pair> {
        public int compare(pair c1, pair c2) {
            return Long.compare(c1.a, c2.a);
        }
    }

    static class PairComparatorval implements Comparator<pair> {
        public int compare(pair c1, pair c2) {
            return Long.compare(c1.b, c2.b);
        }
    }


    static class SReader {
        BufferedReader br;
        StringTokenizer st;

        public SReader() {
            br = new BufferedReader(new
                    InputStreamReader(System.in));
        }

        public static void pr(Object s) {
            System.out.println(s);
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int ni() {
            return Integer.parseInt(next());
        }

        long nl() {
            return Long.parseLong(next());
        }

        double nd() {
            return Double.parseDouble(next());
        }

        String rl() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
    static class SegmentTree
    {
        static int MAX =10000000;
        //Trees For Max query
    static long tree[] = new long[MAX];
    static long lazy[] = new long[MAX];
    //Trees for Min query
    static long tree2[] = new long[MAX];
    static long lazy2[] = new long[MAX];

    static void updateRangeUtil(int si, int ss, int se, int us,
                                int ue, long diff)
    {
        if (lazy[si] != 0)
        {
            tree[si] += lazy[si];
            if (ss != se)
            {
                lazy[si * 2 + 1] += lazy[si];
                lazy[si * 2 + 2] += lazy[si];
            }
            lazy[si] = 0;
        }
        if (ss > se || ss > ue || se < us)
            return;

        if (ss >= us && se <= ue)
        {
            tree[si] += diff;
            if (ss != se)
            {
                lazy[si * 2 + 1] += diff;
                lazy[si * 2 + 2] += diff;
            }
            return;
        }
        int mid = (ss + se) / 2;
        updateRangeUtil(si * 2 + 1, ss, mid, us, ue, diff);
        updateRangeUtil(si * 2 + 2, mid + 1, se, us, ue, diff);
        tree[si] = Math.max(tree[si * 2 + 1], tree[si * 2 + 2]);
    }

    static void updateRangeUtil2(int si, int ss, int se, int us,
                                int ue, long diff)
    {
        if (lazy2[si] != 0)
        {
            tree2[si] += lazy2[si];
            if (ss != se)
            {
                lazy2[si * 2 + 1] += lazy2[si];
                lazy2[si * 2 + 2] += lazy2[si];
            }
            lazy2[si] = 0;
        }
        if (ss > se || ss > ue || se < us)
            return;
        if (ss >= us && se <= ue)
        {
            tree2[si] += diff;
            if (ss != se)
            {
                lazy2[si * 2 + 1] += diff;
                lazy2[si * 2 + 2] += diff;
            }
            return;
        }
        int mid = (ss + se) / 2;
        updateRangeUtil2(si * 2 + 1, ss, mid, us, ue, diff);
        updateRangeUtil2(si * 2 + 2, mid + 1, se, us, ue, diff);
        tree2[si] = Math.min(tree2[si * 2 + 1], tree2[si * 2 + 2]);
    }
    static void updateRange(int n, int us, int ue, long diff)
    {
        updateRangeUtil(0, 0, n - 1, us, ue, diff);
        updateRangeUtil2(0, 0, n - 1, us, ue, diff);
    }
    static long getMaxUtil(int ss, int se, int qs, int qe, int si)
    {
        if (lazy[si] != 0)
        {
            tree[si] += lazy[si];
            if (ss != se)
            {
                lazy[si * 2 + 1] += lazy[si];
                lazy[si * 2 + 2] += lazy[si];
            }
            lazy[si] = 0;
        }
        if (ss > se || ss > qe || se < qs)
            return Long.MIN_VALUE;
        if (ss >= qs && se <= qe)
            return tree[si];
        int mid = (ss + se) / 2;
        return Math.max(getMaxUtil(ss, mid, qs, qe, 2 * si + 1),
                getMaxUtil(mid + 1, se, qs, qe, 2 * si + 2));
    }

    static long getMinUtil(int ss, int se, int qs, int qe, int si)
    {
        if (lazy2[si] != 0)
        {
            tree2[si] += lazy2[si];
            if (ss != se)
            {
                lazy2[si * 2 + 1] += lazy2[si];
                lazy2[si * 2 + 2] += lazy2[si];
            }
            lazy2[si] = 0;
        }
        if (ss > se || ss > qe || se < qs)
            return Long.MAX_VALUE;
        if (ss >= qs && se <= qe)
            return tree2[si];
        int mid = (ss + se) / 2;
        return Math.min(getMinUtil(ss, mid, qs, qe, 2 * si + 1),
                getMinUtil(mid + 1, se, qs, qe, 2 * si + 2));
    }
    static long getMax(int n, int qs, int qe)
    {
        return getMaxUtil(0, n - 1, qs, qe, 0);
    }
    static long getMin(int n, int qs, int qe)
    {
        return getMinUtil(0, n - 1, qs, qe, 0);
    }
    static void constructSTUtil(long arr[], int ss, int se, int si)
    {
        if (ss > se)
            return;
        if (ss == se)
        {
            tree[si] = arr[ss];
            return;
        }
        int mid = (ss + se) / 2;
        constructSTUtil(arr, ss, mid, si * 2 + 1);
        constructSTUtil(arr, mid + 1, se, si * 2 + 2);
        tree[si] = Math.max(tree[si * 2 + 1], tree[si * 2 + 2]);
    }
    static void constructSTUtil2(long arr[], int ss, int se, int si)
    {
        if (ss > se)
            return;
        if (ss == se)
        {
            tree2[si] = arr[ss];
            return;
        }
        int mid = (ss + se) / 2;
        constructSTUtil2(arr, ss, mid, si * 2 + 1);
        constructSTUtil2(arr, mid + 1, se, si * 2 + 2);
        tree2[si] = Math.min(tree2[si * 2 + 1], tree2[si * 2 + 2]);
    }

    static void constructST(long arr[], int n)
    {
        constructSTUtil(arr, 0, n - 1, 0);
        constructSTUtil2(arr, 0, n - 1, 0);
    }
    }
    static class Reader {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public Reader() {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public Reader(String file_name) throws IOException {
            din = new DataInputStream(new FileInputStream(file_name));
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public static void pr(Object s) {
            System.out.println(s);
        }

        public int ni() throws IOException {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        public long nl() throws IOException {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }

        public String rl() throws IOException {
            byte[] buf = new byte[1024]; // line length
            int cnt = 0, c;
            while ((c = read()) != -1) {
                if (c == '\n')
                    break;
                buf[cnt++] = (byte) c;
            }
            return new String(buf, 0, cnt);
        }

        public double nd() throws IOException {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();

            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');

            if (c == '.') {
                while ((c = read()) >= '0' && c <= '9') {
                    ret += (c - '0') / (div *= 10);
                }
            }

            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

        public void close() throws IOException {
            if (din == null)
                return;
            din.close();
        }
    }
}