#include <bits/stdc++.h>
using namespace std;
int a, b, c;
string s;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> a;
  const double pi = 3.14159;
  double ans = (double)2 * pi * a;
  printf("%.6f", ans);
}
