#include <bits/stdc++.h>
clock_t time_p = clock();
using namespace std;
const int N = 1e5 + 10, mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << (2 * acos(-1) * r);
  return 0;
}
