#include <stdio.h>

int sum(int a[], int n) {
	
	int i, sum;

	sum = 0;

	for (i = 0;i < n;i++) {
		sum += a[i];
	}

	return sum;
}

int main(void) {
	int K, N, F;
	int A[500];
	int i;

	scanf("%d%d%d", &K, &N, &F);
	for (i = 0;i < F;i++) scanf("%d", &A[i]);

	if (sum(A, F) > K * N) printf("%d", -1);
	else printf("%d", K * N - sum(A, F));

	return 0;

}