#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int i, n, t, p = INT_MAX, j = 0, x, a = 0, b, l = 0, r = 0, y, k, c,
                         sum = 0, m, d;
  cin >> r;
  double sd = 2 * acos(-1.0) * r;
  printf("%.5f", sd);
  return EXIT_SUCCESS;
}
