public class Main {

    public static void main(String[] args) {

        double pi = 0;

        for (double i=1;i<=100000;i+=2) {

            int x=-1;

            for(double c=0;c<=(i-1)/2;c++){

               x=-1*x;

            }

            pi=pi+x*1/i;

           // System.out.println(x*1/i+",");

        }

        System.out.println(pi*4);

    }

}