public class Qiupai{
	public static void main(String[] args){
		double pi = 0,pi1 = 0,pi2 = 0;
		double m,n;
		for(m = 1;m<1000000000;m+=4)
		{
			pi1 +=1/m;			
		}
		for(n = -3;n>-1000000000;n-=4)
		{
			pi2 +=1/n;
		}
		pi = 4*(pi1 + pi2);
		System.out.printf("%.15f",pi);
	}
}