#include <stdio.h>

void main(void){
	int k = 0;
	int inputK = scanf("%d\n",&k);
	if(inputK == 2){
		printf("Kの入力ミス\n");
	}
	int n = 0;
	int inputN = scanf("%d\n",&n);
	if(inputN == 2){
		printf("Nの入力ミス\n");
	}
	int f = 0;
	int inputF = scanf("%d\n",&f);
	if(inputF == 2){
		printf("Fの入力ミス\n");
	}
	int sum = k*n;
	for(int i = 0; i < f ; i++){
		int a = 0;
		int inputA = scanf("%d\n",&a);
		if(inputA == 2){
			printf("Aの入力ミス\n");
		}		
		sum = sum - a;
		if(sum < 0){
			sum = -1;
			break;
		}
	}
	printf("%d\n",sum);
}