#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int TC;
  TC = 1;
  while (TC--) {
    long double r;
    cin >> r;
    cout << 2 * 3.14159265359 * r << endl;
  }
  return 0;
}
