#include <bits/stdc++.h>
using namespace std;
const int inf = 1001001001;
const int mod = 1000000007;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << 2.0 * r * 3.1415926535 << endl;
  return 0;
}
