#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
long long n;
int main() {
	cin >> n;
	long long ans = n * (n + 1) / 2 - (n / 2) * (n / 2 + 1) - n;
	cout << ans << endl;
}