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.util.*;
import java.io.*;

/**
 *
 * @author usquare
 */

public class ProblemD {
    
    public static int mod = (int) (1e9+7);
    public static InputReader in;
    public static PrintWriter out;
    
    public static void solve() throws FileNotFoundException {
 
        in = new InputReader(System.in);
        out = new PrintWriter(System.out);            
       
        int n=in.ni();
        int ans=(n-1)/2;
        out.println(ans);
        
        out.close();
    }
    
    public static void main(String[] args) {
        new Thread(null ,new Runnable(){
            public void run(){
                try{
                    solve();
                } catch(Exception e){
                    e.printStackTrace();
                }
            }
        },"1",1<<26).start();
    }
    
    static class Pair implements Comparable<Pair>{

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

	Pair (long x,long y){
		this.x=x;
		this.y=y;
	}
        
	public int compareTo(Pair o) {
		if(this.x!=o.x)
                    return Long.compare(this.x,o.x);
		else
                    return Long.compare(this.y,o.y);
		//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;
        }
    
    } 
    
        
    public static long add(long a,long b){
        long x=(a+b);
        while(x>=mod) x-=mod;
        return x;
    }

    public static long sub(long a,long b){
        long x=(a-b);
        while(x<0) x+=mod;
        return x;
    }
    
    public static long mul(long a,long b){
        long x=(a*b);
        while(x>=mod) x-=mod;
        return x;
    }
    public static boolean isPal(String s){
        for(int i=0, j=s.length()-1;i<=j;i++,j--){
                if(s.charAt(i)!=s.charAt(j)) return false;
        }
        return true;
    }
    public static String rev(String s){
		StringBuilder sb=new StringBuilder(s);
		sb.reverse();
		return sb.toString();
    }
    
    public static long gcd(long x,long y){
	if(x%y==0)
		return y;
	else
		return gcd(y,x%y);
    }
    
    public static int gcd(int x,int y){
	if(x%y==0)
		return y;
	else 
		return gcd(y,x%y);
    }
    
    public static long gcdExtended(long a,long b,long[] x){
        
        if(a==0){
            x[0]=0;
            x[1]=1;
            return b;
        }
        long[] y=new long[2];
        long gcd=gcdExtended(b%a, a, y);
        
        x[0]=y[1]-(b/a)*y[0];
        x[1]=y[0];
        
        return gcd;
    }
    

    public static long mulmod(long  a,long b,long m) {
        if (m <= 1000000009) return a * b % m;
 
        long res = 0;
        while (a > 0) {
            if ((a&1)!=0) {
                res += b;
                if (res >= m) res -= m;
            }
            a >>= 1;
            b <<= 1;
            if (b >= m) b -= m;
        }
        return res;
    }
    
    public static int abs(int a,int b){
	return (int)Math.abs(a-b);
    }

    public static long abs(long a,long b){
	return (long)Math.abs(a-b);
    }
    
    public static int max(int a,int b){
	if(a>b)
		return a;
	else
		return b;
    }

    public static int min(int a,int b){
	if(a>b)
		return b;
	else 
		return a;
    }
    
    public static long max(long a,long b){
	if(a>b)
		return a;
	else
		return b;
    }

    public static long min(long a,long b){
	if(a>b)
		return b;
	else 
		return a;
    }

    public static long pow(long n,long p,long m){
	 long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;
	    if(result>=m)
	    result%=m;
	    p >>=1;
	    n*=n;
	    if(n>=m)
	    n%=m;
	}
	return result;
    }
    
    public static long pow(long n,long p){
	long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;	    
	    p >>=1;
	    n*=n;	    
	}
	return result;
    }
     public static void debug(Object... o) {
            System.out.println(Arrays.deepToString(o));
    }
     
    static class InputReader {
    private final InputStream stream;
        private final byte[] buf = new byte[8192];
        private int curChar, snumChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
                this.stream = stream;
        }

        public int snext() {
                if (snumChars == -1)
                        throw new InputMismatchException();
                if (curChar >= snumChars) {
                        curChar = 0;
                        try {
                                snumChars = stream.read(buf);
                        } catch (IOException e) {
                                throw new InputMismatchException();
                        }
                        if (snumChars <= 0)
                                return -1;
                }
                return buf[curChar++];
        }

        public int ni() {
                int c = snext();
                while (isSpaceChar(c)) {
                        c = snext();
                }
                int sgn = 1;
                if (c == '-') {
                        sgn = -1;
                        c = snext();
                }
                int res = 0;
                do {
                        if (c < '0' || c > '9')
                                throw new InputMismatchException();
                        res *= 10;
                        res += c - '0';
                        c = snext();
                } while (!isSpaceChar(c));
                return res * sgn;
        }

        public long nl() {
                int c = snext();
                while (isSpaceChar(c)) {
                        c = snext();
                }
                int sgn = 1;
                if (c == '-') {
                        sgn = -1;
                        c = snext();
                }
                long res = 0;
                do {
                        if (c < '0' || c > '9')
                                throw new InputMismatchException();
                        res *= 10;
                        res += c - '0';
                        c = snext();
                } while (!isSpaceChar(c));
                return res * sgn;
        }

        public int[] nextIntArray(int n) {
                int a[] = new int[n];
                for (int i = 0; i < n; i++) {
                        a[i] = ni();
                }
                return a;
        }

        public long[] nextLongArray(int n) {
                long a[] = new long[n];
                for (int i = 0; i < n; i++) {
                        a[i] = nl();
                }
                return a;
        }
        
        public String readString() {
                int c = snext();
                while (isSpaceChar(c)) {
                        c = snext();
                }
                StringBuilder res = new StringBuilder();
                do {
                        res.appendCodePoint(c);
                        c = snext();
                } while (!isSpaceChar(c));
                return res.toString();
        }

        public String nextLine() {
                int c = snext();
                while (isSpaceChar(c))
                        c = snext();
                StringBuilder res = new StringBuilder();
                do {
                        res.appendCodePoint(c);
                        c = snext();
                } while (!isEndOfLine(c));
                return res.toString();
        }

        public boolean isSpaceChar(int c) {
                if (filter != null)
                        return filter.isSpaceChar(c);
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        private boolean isEndOfLine(int c) {
                return c == '\n' || c == '\r' || c == -1;
        }

        public interface SpaceCharFilter {
                public boolean isSpaceChar(int ch);
        }
        
    }
}    

