I'm trying to grasp the use of functions with the cs50 library to complete the pset1 water task. My code will outright ignore the function I've written, I think this is due to line 10, but what am I missing?

#include <stdio.h>
#include <cs50.h>

//a function that takes 'a', multiplies that by 16 and divides 192 from this
int convert(int a);

int main(void)
{
    printf("How many minutes do you spend in the shower: ");
    int a = GetInt();

    if(a <= 0)
    {
        printf("You don't shower? Nasty./n");
        return 0;
    }
    else
    {
        printf("You use %i bottles of water per shower!\n", a);
    }
}

/* Function Argument */
int convert(int a)
{
    return 192/(16*a);
}

Many Thanks