#include <bits/stdc++.h>
using namespace std;
int main() {
  int R;
  float circ;
  cout << "Enter radio: ";
  cin >> R;
  if (R >= 1 && R <= 100) {
    circ = 2 * R * 3.14159265;
    cout << "Result is: " << circ;
  }
  return 0;
}
