#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int inf = 1e9;
const long long INF = 1e18;
const double pi = 3.14159265358979323846;
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << 2.0 * pi * r << endl;
}
