#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
const double EPS = 1e-10;
const int MAX = 200000;
int main() {
  double n;
  cin >> n;
  cout << setprecision(5) << n * (double)2 * 3.14159265359 << endl;
  return 0;
}
