#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 11;
const int M = 1e6 + 11;
const int MOD = 1e9 + 7;
const double PI = acos(-1.0);
int main(int argc, char **args) {
  freopen("in.txt", "r", stdin);
  double r;
  cin >> r;
  cout << 2 * PI * r << "\n";
  return 0;
}
