#include <bits/stdc++.h>
using namespace std;
float round(float var) {
  float value = (int)(var * 100 + .5);
  return (float)value / 100;
}
int main() {
  int n = 0;
  std::cin >> n;
  float var = n * 3.14159265358;
  cout << round(var);
  return 0;
}
