#include<bits/stdc++.h>

using namespace std;

int main() {
    int n;
    while (cin >> n) {
        double s = 0;
        for (int i = 1; i <= n; i += 2) {
            if (i % 4 == 1)
                s += 1.0 / i;
            else
                s -= 1.0 / i;
        }
        cout << fixed << 4 * s << endl;
    }
    return 0;
}