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

int main(void)
{
	int k, n, f;
	int total;
	int *p;
	int i;

	do {
		scanf("%d", &k);
	} while (k < 1 || k > 1000);

	do {
		scanf("%d", &n);
	} while (n < 1 || n > 1000);

	do {
		scanf("%d", &f);
	} while (f < 1 || f > 100);

	total = k * n;

	p = malloc(f * sizeof(int));
	if (p == NULL)
		printf("記憶域の確保に失敗しました。\n");
	else {
		for (i = 0; i < f; i++) {
			do {
				scanf("%d", &p[i]);
			} while (p[i] < 1 || p[i] > 100);
		}

		for (i = 0; i < f; i++) {
			total -= p[i];

			if (total < 0) {
				printf("%d", -1);
				break;
			}
			if (i == f - 1)
				printf("%d", total);
		}

		free(p);
	}

	return 0;
}