#include <bits/stdc++.h>
using namespace std;
string alphabet = "abcdefghijklmnopqrstuvwxyz";
string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
int main() {
  int r;
  r = 73;
  double pie = 3.14159265358979;
  cout << setprecision(10) << 2 * r * pie << endl;
}
