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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

（i+j）mod（n+1） 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

题意：就是说有n个地点，每来往i，j两个地点就花费（i+j）mod（n+1），问遍历所有地点的最低花费

当i+j==n+1每两地花费最小，如1,n 2，n-1 。。。
只需以1->n->2->n-1->3...这样的顺序花费就最小。

#include<stdio.h>
int main(){
    long long n;
    scanf("%I64d",&n);
    if(n%2==0){
        printf("%I64d",n/2-1);
    }
    else printf("%I64d",(n-1)/2);
}