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: Accepted)

#include <iostream>
#include <vector>
#include <string.h>
#include <set>
#include <map>
#include <assert.h>
#include <algorithm>
#include <queue>
#include <bitset>
#include <stack>
#include <chrono>
#include <random>
#define all(x) x.begin(),x.end()
#define ff first
#define ss second
#define ll long long
#define MOD 1000000007 
#define INF 1000000000000000000
#define uid uniform_int_distribution <int>
#define rnd mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define FIO ios_base::sync_with_stdio(false); cin.tie(NULL);
using namespace std;
// Read the question carefully and see all the given sample tests
// Think about the correctness before implementing

int main()
{
	int n;
	cin >> n;
	if(n % 2 == 0)
	{
		cout << n / 2 - 1;
	}
	else
	{
		cout << n / 2;
	}
}