#include <stdio.h>

int main(void) {
	int i, K, N, F, year, sum = 0;
	scanf("%d%d%d", &K, &N, &F);
	for(i = 1;i <= F;i++){
		scanf("%d", &year);
		sum += year;
	}
	printf("%d\n", (K * N - sum >= 0)?K * N - sum:-1);
	
	return 0;
}