#include <iostream>
#include <vector>
#include <algorithm>
#include <string>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)
#define dump(val) cerr << __LINE__ << ":\t" << #val << " = " << (val) << endl

using namespace std;

typedef long long int lli;

int main() {
    lli L;
    cin >> L;
    cout << L * (L - 1) / 2 - (L - 1) * (L + 1) / 4 << endl;
    return 0;
}