public class Main {

 public static void main(String[] args) {

  double pi = 0;

  for ( int i = 1; i < 10000000; i += 2) {

   if ((i + 1) / 2 % 2 == 0)

    i = -i;

   pi += (1.0 / i);

   i = Math.abs(i);

  }

  pi*=4;

  System.out.println(pi);

 }

}