#include <bits/stdc++.h>
using namespace std;
const int maxn = 100010;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout << setprecision(20);
  cout << fixed;
  int r = 0;
  cin >> r;
  cout << (2.0 * M_PI * r) << endl;
}
