#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const double PI = acos(-1.0);
int main() {
  ios_base::sync_with_stdio(false);
  int r;
  cin >> r;
  cout << fixed << setprecision(2) << (double)r * 2 * PI << '\n';
  return 0;
}
