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>.
    import java.awt.Point;
import java.io.*;
        		    import java.math.*;
        		    import java.util.*;
        		    import javax.print.attribute.SetOfIntegerSyntax;


     
        		    // kishanthesiya25@gmail.com
        		    // kishan_25
        		    // @daiict
        		    
     
        		    public class latest{
        		    	private static InputStream stream;
        		    	private static byte[] buf = new byte[1024];
        		    	private static int curChar;
        		    	private static int numChars;
        		    	private static SpaceCharFilter filter;
        		    	private static PrintWriter pw;

        		    	
        		    	static ArrayList<Integer> arr[];
        		    	static ArrayList<Integer> ans;
        		    	static int f;
        		    	
        		    	private static long count = 0,mod=1000000007;
        		    //	private static TreeSet<Integer> ts=new TreeSet[200000];
     
        		        public static void main(String[] args) {
        		        	InputReader(System.in);
        		    		pw = new PrintWriter(System.out); 
        		            new Thread(null ,new Runnable(){
        		               public void run(){
        		                   try{
        		                       soln2();
        		                       pw.close();
        		                   } catch(Exception e){
        		                       e.printStackTrace();
        		                   }
        		               }
        		           },"1",1<<26).start();
        		       }

        		    	public static long gcd(long x, long y) {
        		    		if (x == 0)
        		    			return y;
        		    		else
        		    			return gcd( y % x,x);
        		    	}	public static boolean isPrime(int n) {
        		    	// Corner cases
			    		if (n <= 1)
			    			return false;
			    		if (n <= 3)
			    			return true;
			     
			    		// This is checked so that we can skip 
			    		// middle five numbers in below loop
			    		if (n % 2 == 0 || n % 3 == 0)
			    			return false;
			     
			    		for (int i = 5; i * i <= n; i = i + 6)
			    			if (n % i == 0 || n % (i + 2) == 0)
			    				return false;
			     
			    		return true;
			    	}
        		        static LinkedList<Integer> adj[];
        		      
        		        static boolean Visited[];
        		        static long color[];
        		        static long dist[][];
        		        static long no_sp_no_sub[];
        		        static int visited[];
        		    	static Stack<Integer> stack;
        		    	static int n;
        		    	static int m;
        		    	static int main[];
        		    	static int status[];
        		        
        		        private static void soln2(){
        		        	
        		        	Scanner in = new Scanner(System.in);
        		      
        		        	int n=nextInt();
        		    	
        		        	if(n%2 == 0){
        		        		System.out.println(n/2 - 1);
        		        	}else{
        		        		System.out.println(n/2);
        		        	}
        		    		
        		        }
        		        

        		    	public static void dfs(int i){
        		    		
        		    		if(status[i] == 0){
        		    			status[i] = 1;

            		    		 for(int j = 0;j<arr[i].size();j++){
            		    			dfs(arr[i].get(j));
            		    		}
            		    		
            		    		status[i] = 2;
            		    		ans.add(i);
            		    		
        		    		}else if(status[i] == 1){
        		    			f = 1;
        		    			return;
        		    		}
        		    	}
        		      
        		    
        		        
        		        
        		        private static long maxSubarraySum(long a[]){
        		        	long max_so_far=Long.MIN_VALUE;
        		        	long max_ending_here=0;
        		        	for(int i=0;i<a.length;i++){
        		        		max_ending_here+=a[i];
        		        		if(max_ending_here>max_so_far)
        		        			max_so_far=max_ending_here;
        		        		if(max_ending_here<0)
        		        			max_ending_here=0;
        		        	}
        		        	return max_so_far;
        		        }

     
        		    	// To Get Input
        		    	// Some Buffer Methods
     
        		    	public static void InputReader(InputStream stream1) {
        		    		stream = stream1;
        		    	}
     
        		    	private static boolean isWhitespace(int c) {
        		    		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        		    	}
     
        		    	private static boolean isEndOfLine(int c) {
        		    		return c == '\n' || c == '\r' || c == -1;
        		    	}
     
        		    	private static int read() {
        		    		if (numChars == -1)
        		    			throw new InputMismatchException();
        		    		if (curChar >= numChars) {
        		    			curChar = 0;
        		    			try {
        		    				numChars = stream.read(buf);
        		    			} catch (IOException e) {
        		    				throw new InputMismatchException();
        		    			}
        		    			if (numChars <= 0)
        		    				return -1;
        		    		}
        		    		return buf[curChar++];
        		    	}
     
        		    	private static int nextInt() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		int sgn = 1;
        		    		if (c == '-') {
        		    			sgn = -1;
        		    			c = read();
        		    		}
        		    		int res = 0;
        		    		do {
        		    			if (c < '0' || c > '9')
        		    				throw new InputMismatchException();
        		    			res *= 10;
        		    			res += c - '0';
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res * sgn;
        		    	}
     
        		    	private static long nextLong() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		int sgn = 1;
        		    		if (c == '-') {
        		    			sgn = -1;
        		    			c = read();
        		    		}
        		    		long res = 0;
        		    		do {
        		    			if (c < '0' || c > '9')
        		    				throw new InputMismatchException();
        		    			res *= 10;
        		    			res += c - '0';
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res * sgn;
        		    	}
     
        		    	private static String nextToken() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		StringBuilder res = new StringBuilder();
        		    		do {
        		    			res.appendCodePoint(c);
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res.toString();
        		    	}
     
        		    	private static String nextLine() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		StringBuilder res = new StringBuilder();
        		    		do {
        		    			res.appendCodePoint(c);
        		    			c = read();
        		    		} while (!isEndOfLine(c));
        		    		return res.toString();
        		    	}
     
        		    	private static int[] nextIntArray(int n) {
        		    		int[] arr = new int[n];
        		    		for (int i = 0; i < n; i++) {
        		    			arr[i] = nextInt();
        		    		}
        		    		return arr;
        		    	}
     
        		    	private static int[][] next2dArray(int n, int m) {
        		    		int[][] arr = new int[n][m];
        		    		for (int i = 0; i < n; i++) {
        		    			for (int j = 0; j < m; j++) {
        		    				arr[i][j] = nextInt();
        		    			}
        		    		}
        		    		return arr;
        		    	}
        		    	private static char[][] nextCharArray(int n,int m){
        		    		char [][]c=new char[n][m];
        		    		for(int i=0;i<n;i++){
        		    			String s=nextLine();
        		    			for(int j=0;j<s.length();j++){
        		    				c[i][j]=s.charAt(j);
        		    			}
        		    		}
        		    		return c;
        		    	}
     
        		    	private static long[] nextLongArray(int n) {
        		    		long[] arr = new long[n];
        		    		for (int i = 0; i < n; i++) {
        		    			arr[i] = nextLong();
        		    		}
        		    		return arr;
        		    	}
     
        		    	private static void pArray(int[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static void pArray(long[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static void pArray(boolean[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static boolean isSpaceChar(int c) {
        		    		if (filter != null)
        		    			return filter.isSpaceChar(c);
        		    		return isWhitespace(c);
        		    	}
     
        		    	private interface SpaceCharFilter {
        		    		public boolean isSpaceChar(int ch);
        		    	}
     
     
    }
        		    
        		    class nodi{
        		    	int a;
        		    	int b;
        		    }
        		    
        		    class MyComp2 implements Comparator<nodi>{
        		        
        		    	@Override
        		    	public int compare(nodi o1, nodi o2) {
        		    		if(o1.b>o2.b){
        		    			return 1;
        		    		}else if(o1.b<o2.b){
        		    			return -1;
        		    		}else{
        		    			return 0;
        		    			
        		    			
        		    		}
        		    		
        		    		
        		    	}
        		    }
        		    
        		    class node{
        		    	int i;
        		    	int a;
        		    	int b;
        		    }
        		    
        		    class MyComp implements Comparator<node>{
        		        
        		    	@Override
        		    	public int compare(node o1, node o2) {
        		    		if(o1.a>o2.a){
        		    			return 1;
        		    		}else if(o1.a<o2.a){
        		    			return -1;
        		    		}else{
        		    			return 1;	
        		    		}
        		    		
        		    		
        		    	}
        		    }
        		    
        		    class Node{
        		    	int to;
        		    	long dist;
        		    	Node(int to,long dist){
        		    		this.to=to;
        		    		this.dist=dist;
        		    	}
        		    	
        		    }
        		  
        		    class Pair implements Comparable<Pair>{

        		        long x,y;
        		        int i;
        		        
        			Pair (long x,long y,int i){
        				this.x=x;
        				this.y=y;
        				this.i=i;
        			}

        			public int compareTo(Pair o) {
        		            return Long.compare(this.x,o.x);
        				//return 0;
        			}

        		            public boolean equals(Object o) {
        		            if (o instanceof Pair) {
        		                Pair p = (Pair)o;
        		                return p.x == x && p.y == y && p.i==i;
        		            }
        		            return false;
        		        }

        		        @Override
        		        public String toString() {
        		            return x+" "+y+" "+i;
        		        }
        		        
        		    } 
        		    
        		    
        		    class Dsu{
	    		    	private int rank[], parent[] ,n;
	    		    	private static int[] parent1;
	    		    	Dsu(int size){
	    		    		this.n=size+1;
	    		    		rank=new int[n];
	    		    		//parent=new int[n];
	    		    		parent=new int[n];
	    		    	makeSet();
	    		
	    		    	}
	    		    	
	    		    	void makeSet(){
	    		    		for(int i=0;i<n;i++){
	    		    			parent[i]=i;
	    		    		}
	    		    	}
	    		    	
	    		    	int find(int x){
	    		    		if(parent[x]!=x){
	    		    			
	    		    			parent[x]=find(parent[x]);
	    		    		}
	    		    		return parent[x];
	    		    	}
	    		    
	    		    	
	    		    	boolean union(int x,int y){
	    		    		int xRoot=find(x);
	    		    		int yRoot=find(y);
	    		    		
	    		    		if(xRoot==yRoot)
	    		    			return false;
	    		    		if(rank[xRoot]<rank[yRoot]){
	    		    		parent[xRoot]=yRoot;	
	    		    		}else if(rank[yRoot]<rank[xRoot]){
	    		    			parent[yRoot]=xRoot;
	    		    		}else{
	    		    			parent[yRoot]=xRoot;
	    		    			rank[xRoot]++;
	    		    		}
	    		    		return true;
	    		    	}

	    		     
	    		    }
  
