A Monte Carlo Integral calculator

mcint

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

double f(double x)
{
  return exp(x) * x;
}

double random(int fs, int ls)
{
  return ((rand() / (double) RAND_MAX) *  (ls - fs)) + fs;
}

int main()
{
  srand(time(NULL));
  int N, it, b, a, i, j;
  // For a = 2 and b = 5 and f(x) = x * e^x
  double realvalue = 586.264;
  printf("Input number of iterations: ");
  scanf("%d", &it);
  printf("Input number of samples: ");
  scanf("%d", &N);
  printf("Input the boundaries (a, b): ");
  scanf("%d %d", &a, &b);

  double sum = 0, ans = 0;
  // Monte Carlo Method Calculation
  for (i = 0; i < it; i++)
    for (j = 0; j < N; j++)
    {
      double vl = random(a, b);
      sum += f(vl);
    }
  sum /= it;
  sum = (sum * (b - a)) / N;

  // Midpoint Rule Calculation
  double dif = (b - a) / ((double)N);
  for (j = 0; j < N; j++)
  {
    double vl = j * dif + a;
    ans += f(vl) * dif;
  }

  printf("Real Value: %lf\n", realvalue);
  printf("Integral: %lf, error: %0.3lf%%\n", sum, (100 * fabs(sum - realvalue) / realvalue));
  printf("Integral (using the midpoint rule): %0.3lf, error %0.3lf%%\n", ans, (100 * fabs(ans - realvalue) / realvalue));
}