Loop counter/average question

[Wynd]

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:

#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;
}