#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t = 1;
  while (t--) {
    double n;
    cin >> n;
    double ans = (n * 2) * 3.14159;
    cout << ans;
  }
}
