#include <bits/stdc++.h>
using namespace std;
const long long M = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  float r;
  cin >> r;
  float pi = 3.141592;
  pi = 2.0 * pi * r;
  cout << pi;
  return 0;
}
