#include <bits/stdc++.h>
constexpr auto NMAX = (int)2e5 + 20;
constexpr auto INF = 2000000000;
using namespace std;
int r;
double const pi = 3.14159;
int main() {
  cin >> r;
  cout << 2 * pi * r << '\n';
  return 0;
}
