#include <stdio.h>

int main() {
	int k, n, f, a[100], b = 0, i;

	scanf("%d %d %d", &k, &n, &f);
	for (i = 0; i < f; i++) {
		scanf("%d", &a[i]);
		b += a[i];
	}
	if (b > k*n) printf("-1\n");
	else printf("%d\n", k*n - b);

	return 0;
}