#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) { return a * (b / gcd(a, b)); }
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  double n;
  cin >> n;
  cout << (n * 2) * M_PI << "\n";
}
