Loop counter/average question

Ok, I have to write this program for school. I'm almost done, but I need help on one part. The program runs a loop 20 times and collects A, P, C, P, P. I have that part, but I also need to find the average number of times it took. This is what I have so far:

Code:

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#include <iostream>
#include <cmath>
#include <cstdlib>
using namespace std;

int prem();
int average(int, int);

int main()
{
        srand(time(NULL));
        int a = 0, c = 0, p = 0, sum = 0, counter = 0;
       
        for (int i = 1; i < 20; i++)
        {
                while (a < 1 || c < 1 || p < 3)
                {
                        switch (prem())
                        {
                        case 1:
                                a++;
                                break;
                        case 2:
                                c++;
                                break;
                        case 3:
                        case 4:
                                p++;
                                break;
                        }
                        cout<<"A: "<<a<<" C: "<<c<<" P: "<<p<<" Sum: "<<endl;
                }

                cout<<"Took "<<a + c + p<<" times to get APCPP"<<endl;
                a = c = p = 0;
        }

        cout<<"On average, it took "<<average(sum, counter)<<" boxes."<<endl;

        getchar();
        return 0;
}

int prem()
{
        return 1 + (rand() % 4);
}

int average(int sum, int counter)
{
        return sum;
}

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Average=sum of elements / amount of elements

I know that, I just wasn't sure how to implement that into the program. I figured it out though.