Why the user-defined function needs to be before the main function for a function pointer to work

Question

IN the following program why the user-defined function needs to be before the main function in order to run the program. When I put the function after the main function, it doesn't work. Why?

This one works:

#include <stdio.h>

int add(int a, int b)
{
    int c;

    c = a + b;

    return c;
}

int main()
{
    int a, b, sum;
    int (*ab)(int, int) = &add; // Declaration and initialization of function pointer

    sum = (*ab)(1, 2); // Dereferencing of function pointer

    printf("1 + 2 = %d", sum);

    return 0;
}

But why this one doesn't work?

#include <stdio.h>

int main()
{
    int a, b, sum;
    int (*ab)(int, int) = &add; // Declaration and initialization of function pointer

    sum = (*ab)(1, 2); // Dereferencing of function pointer

    printf("1 + 2 = %d", sum);

    return 0;
}

int add(int a, int b)
{
    int c;

    c = a + b;

    return c;
}