#include <stdio.h>
#include <stdlib.h>

int main()
{
	int k, n, f;

	scanf("%d%d%d", &k, &n, &f);

	int s = k * n;

	int* w;
	w = (int*)malloc(sizeof(int) * f);

	for (int i = 0; i < f; i++)
	{
		scanf("%d", &w[i]);
		s -= w[i];
	}


	if (s >= 0)
	{
		printf("%d", s);
	}
	else
	{
		printf("-1");
	}

	return 0;
}