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.io.*;
import java.util.*;
public class Solution{  
      
      static int mod=1000000007;
    

      public static void main(String[] args){ 

                     int n = intt();
                         n = (n-1)/2;
                      System.out.println(n);


                                                                                                                           
    } 

         static FastReader sc=new FastReader();
         static PrintWriter out = new PrintWriter(System.out);
            static int intt(){
                       int x = sc.nextInt();
                       return(x);
                }
               static long longg(){
                          long x = sc.nextLong();
                           return(x);
                }
               static double doublee(){
                          double t = sc.nextDouble();
                           return(t);
                }
              static String str(){
                       String s = sc.next();
                         return(s);
                }
              static String strln(){

                        String s = sc.nextLine();
                           return(s);
              }
              static long pow(long a,long b){
                       long ans=1;
                      while(b>0){
                          if(b%2==1)
                              ans=(ans*a);
                              a=(a*a)%mod;
                              b=b/2;
                      }
                    return(ans);
                }
             static long GCD(long a,long b){
                           if(b%a==0)
                               return(a);
                               return(GCD(b%a,a));
              }
            static long LCM(long a,long b){
                         return ((a*b)/GCD(a,b));
               }

                static long abs(long x,long y)
                   { return(Math.abs(x-y));}

              static long min(long x,long y){
                     return(Math.min(x,y));
                 }

              static long max(long a,long b){
                        return Math.max(a,b);
                  } 

              static long Fermat(long a,long p){
                           long b = p-2,ans=1;
                            while(b>0){
                                 if(b%2==1)
                                      ans=(ans*a)%p;
                                       a=(a*a)%p;
                                       b=b/2;
                              }
                           return(ans);
                   }
              static void Fact(long a[],long m){
                              a[0]=1;
                           for(int i=1;i<a.length;i++)
                                   a[i]=(i*a[i-1])%m;

                      }
              static void Fact_Inv(long a[],long F[],long m){

                                 int n =a.length;
                                     a[n-1]=Fermat(F[n-1],m);
                                 for(int i=n-2;i>=0;i--)
                                        a[i]=((i+1)*a[i+1])%m;
                        }
               /*static  long d, x, y;
              static void extendedEuclid(long A, long B) {
                            if(B == 0) {
                                  d = A;
                                  x = 1;
                                  y = 0;
                              }
                           else {
                                extendedEuclid(B, A%B);
                                 long temp = x;
                                  x = y;
                                 y = temp - (A/B)*y;
                            }
                     } */
                 /*static class pair implements Comparable<pair>{
                       int x,y;
                      pair(int a,int b){
                             x=a;
                             y=b;
                       }
                     public int compareTo(pair p1)
                           { return((p1.x-p1.y)-(this.x-this.y));}
          
                   public boolean equals(Object obj) {
                         if (this == obj)
                                return true;
                        if (obj == null)
                                return false;
                        if (getClass() != obj.getClass())
                                return false;
                         pair other = (pair) obj;
                          if (x != other.x)
                                  return false;
                          if (y != other.y)
                                  return false;
                            return true;
                      }
                      @Override
                 public int hashCode() {
                      return(x-y);
                   } 
          }   */
   static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
}