#include <stdio.h>

#include <stdlib.h>

#include <time.h>

#include <math.h>

int main() {

    double x, pi;

    double p=0.0, p2=0.0, e=0.0;                //p=f(r),p2=f^2,e=error

    int try, ntry;

    printf("Input the number of MC trials\n");

    scanf("%d",&ntry);

    srand((unsigned)time((long *)0));

    for (try=0; try<ntry; try++) {

        x = rand()/(double)RAND_MAX;

        p += 4.0/(1.0 + x*x);

        p2 += 4.0* 4.0/((1.0 + x*x)*(1.0 + x*x));

    }

    pi = p/ntry;

    p2 = p2/ntry;

    e  = sqrt((p2-pi*pi)/((double)(ntry-1)));

 

    printf("MC estimate for PI = %f\n Error = %e\n",pi,e);

    return 0;

}