#include <bits/stdc++.h>
using namespace std;
const int IINF = 1e9;
const long long LINF = 1e18;
const long long MOD = 1e9 + 7;
int main() {
  double R;
  cin >> R;
  printf("%.5f\n", 2 * R * M_PI);
  return 0;
}
