#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
int main() {
  double pi = 3.1415926535897932384626433832795;
  double n;
  cin >> n;
  printf("%.10f\n", n * 2.0 * pi);
  return 0;
}
