Problem:
Score : 100 points

Problem Statement

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

1 \leq N \leq 10^6

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
#include<algorithm>
using namespace std;
#define _GLIBCXX_DEBUG
#define acc_io cin.tie(0);ios::sync_with_stdio(false)
#define MOD 1e9+7
#define ll long long
#define all(x) x.begin(),x.end()
#define vvl vector<vector<ll>>
#define vl  vector<ll>
#define rep(i,sta,end) for(ll i=sta;i<end;i++)
//name[i]   -> vl  name(i)
//name[i][j]-> vvl name(i,vl(j))
//or-> ||

signed main(){
	acc_io;
	ll n;
	cin>>n;
  	if(n%2==0)n++;
	cout<<(n/2-1)<<endl;
	return 0;
}