#include <bits/stdc++.h>
const unsigned long long int mod = 1e9 + 7;
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  double r;
  cin >> r;
  cout << fixed << setprecision(15) << r * 2 * 3.14159265358979 << "\n";
}
