#include <bits/stdc++.h>
using namespace std;
const int INF = 2000000000;
const double pi = 3.1415926;
const int base = 1000000007;
double n;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout.precision(25);
  cout.setf(ios_base::fixed);
  cin >> n;
  cout << 2 * pi * n;
  return 0;
}
