#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;
const int ninf = 0xc0c0c0c0;
int main() {
  double n;
  cin >> n;
  const double pi = acos(-1);
  printf("%.4lf\n", 2 * pi * n);
  return 0;
}
