#include "stdio.h"
#include "math.h"
double Pi(int debug){
	double a=1, b= sqrt(2.0)/2, s=0.25, t=1, befor, dab=a-b;
	do{
		double aa=a;
		befor=dab; a = (a+b)*0.5; b=sqrt(aa * b); aa = a - aa; dab = a-b;
		s -= t * aa * aa; t *= 2.0;
		if(debug)printf("\na=%19.16fl b=%19.16fl a-b=%10.4el s=%10.4el", a, b,dab,s);
		
	}while(dab != befor);
	t=a+b; return 0.25 * t * t / s;
}
int main(){
	printf("\n%19.16fl", Pi(true));
	getchar();
	return 0;
}