public class Main {

    public static void main(String[] args) {

        double pi4 = 0;

        for (int i=0;i<100000;i++) {

            double m=Math.pow(-1,i);

            double n=2*i+1;

            pi4+=m*(1/n);

        }

        System.out.println(pi4*4);

    }

}