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


int main() {

    int r;
    cin >> r;

    double ans = 2 * M_PI * r;
    cout << std::fixed << std::setprecision(15) << ans << endl;



    return 0;
}

