#include <bits/stdc++.h>
//#include <iostream>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

#define PI 3.1415926535

int main() {
    int N;
    cin >> N;

    cout << (double)N * (double)2.0 * (double)PI;

}
