#include <stdio.h>

int main(void)
{
	int k, n, f, a, i, s = 0;
	scanf("%d%d%d", &k, &n, &f);
	for (i = 0; i < f; i++) {
		scanf("%d", &a);
		s += a;
	}
	f = k * n - s < 0 ? -1 : k * n - s;
	printf("%d\n", f);
	return 0;
}