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

int main()
{
	int K, N, F, a, A;
	scanf("%d%d%d", &K, &N, &F);
	a = K * N;
	for (int i = 0; i < F; i++)
	{
		scanf("%d", &A);
		a -= A;
	}
	if (a < 0)
		a = -1;
	printf("%d\n", a);
	return 0;
}