#include <bits/stdc++.h>
using namespace std;
const double PI = 3.141592653589793238460;

#define ll long long

int main(){

    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int n;
    cin >> n;
    cout << setprecision(20) << 2*n*PI << "\n";

    return 0;
}