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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 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 .

从1到n相互对称的城市是免费的而右边到左边相邻的需要一块钱如n=10，从1到10不花钱
10到2一块钱以此类推 只需要个数/2的钱。

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin>>n;
    if(n%2==0)
    cout<<n/2-1;
    else
    cout<<n/2;
}