#include <stdlib.h>
#include <stdio.h>

int main(void) {
    int j=0;
    double result=0;
    /* approximation of pi/4 using leibniz' series */
    while (j<2000000000) {
	result=result+1.0/(2.0*(double)j+1.0);
	j++;
	result=result-1.0/(2.0*(double)j+1.0);
	j++;
    }
    printf("%.16f\n",4.0*result);
    return 0;
}