#include <iostream>
#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
#define rep(i,n) for(int i = 0; i < ((int)(n)); i++)   // 0-indexed昇順

int main() {
    int R;
    cin >> R;
    std::cout << std::fixed << std::setprecision(15);
    cout << 2 * M_PI * R << endl;
}