Check if a number is prime or not

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

int max, num;
int *start, *primes;

int prime(int i) {
  if(num%primes[i] == 0)
    return 0;
  if(i<max)
    prime(++i);
  return 1;
}

int main(void) {
  int i, j;

  printf("Insert the number: ");
  scanf("%d", &num);

  if(num < 2) {
    printf("You must insert a number greater than 1.");
    return -1;
  } else if(num == 2) {
    printf("%d is prime.", num);
    return 0;
  } else if(num%2 == 0) {
    printf("%d is NOT prime.", num);
    return 1;
  }

  max = ((int)(sqrt(num)-0.5)%2 == 0) ? (int)(sqrt(num)-0.5)/2 : ((int)(sqrt(num)-0.5)-1)/2;
  start = calloc(max, sizeof(int));
  for(i=0; i<max; i++)
    start[i] = (i*2)+3;

  for(i=0; i<max; i++) {
    if(start[i] == 0)
      continue;
    for(j=(i+1); j<max; j++)
      if(start[j] != 0 && start[j]%start[i] == 0)
        start[j] = 0;
  }

  for(i=0, j=0; i<max; i++)
    if(start[i] != 0) {
      primes = realloc(primes, j+1);
      primes[j++] = start[i];
    }

  if(prime(0))
    printf("%d is prime.", num);
  else
    printf("%d is NOT prime.", num);

  free(start);
  free(primes);
  return 0;
}