#include<stdio.h>	
int main(void){	

	int 	i = 0;
	double  Sum = 0.0, err, res;		

	printf("Input error:");
	scanf("%lf",&err);

	while(1){

		if(i%2==0)
			Sum = Sum + (4.0/(2.0*i+1.0));
		else
			Sum = Sum - (4.0/(2.0*i+1.0));
		
		res = Sum - 3.1415;

		if(-err<res && res<err){ 
			printf("N is %d.\n",i); 
			break;
		}
		i++;
	}	

	return 0;

}