#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 60;
signed main() {
  long long x = 0, y = 0, z = 0, w = 0, n = 0, m = 0, o = 0;
  std::cin >> x;
  std::cout << x * 2 * M_PI << '\n';
}
