#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t;
  t = 1;
  while (t--) {
    float r;
    cin >> r;
    float pi = 3.141592653589793238;
    cout << fixed << setprecision(9) << 2 * pi * r;
  }
}
