public class Main {
    public static void main(String[] args) {
        double pi = 1;
        int i = 2;
        for (int n=3;n<1000;n=n+2) {
            // TODO            
            pi=pi-(float)(Math.pow(-1,i)/n);
            i=i+1;
        }
        System.out.println(4*pi);
    }
}