Тип возвращаемого значения - C (СИ)

#include <stdio.h>
 
int main()
{
    unsigned long n;
    printf("n = ");
    scanf("%u", &n);
 
    unsigned long a, b;
    for (a = 2; a <= n; a++) {
        b = sum_div(a);
        if (a < b && a == sum_div(b))
            printf("%d   %d\n", a, b);
    }
}
 
unsigned long sum_div(unsigned long num)
{
    unsigned long i, sum = 1, lim = sqrt(num);
    short d = (num % 2) ? 2 : 1;
    for (i = 1 + d; i <= lim; i += d)
        if (num % i == 0)
            sum += i + num / i;
    return sum;
}