#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int TESTS = 1;
  while (TESTS--) {
    double r;
    cin >> r;
    cout << setprecision(5) << 3.1415 * r;
  }
  return 0;
}
