Why does this program crash?

#include <stdio.h>

int main(){
   
    int i, j, num;
   
    scanf("%d", num);
    for(i=2;i<num;i++){
    j=i;
    if(num%j==0){
    printf("The number is not prime.\n");
    break;}
    else
    printf("The number is prime.\n");
}

system("pause");

return 0;
}

>>15
I think it's because we have a soft spot for Cee, if he'd used #include<iostream>, cout, cin, etc I imagine he would have received an ungodly amount of abuse.