#include <bits/stdc++.h>
using namespace std;
const int inf = 2e9 + 500;
const int N = 1e3 + 10;
const int bits = 20;
const long double pi = 3.14159265358979323846;
const long long mod = 1e9;
const long long p = 29;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  std::cout << std::fixed << std::setprecision(20);
  ;
  long double r;
  cin >> r;
  cout << 2.0 * pi * r << "\n";
}
