#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;
const long long MOD = 1000000007;
const int MAX = 510000;
int main() {
  double R;
  cin >> R;
  double pi = 3.14159265358979;
  cout << setprecision(20) << fixed;
  cout << 2 * pi * R << endl;
}
