import java.util.Scanner;
public class No0143 {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int sum = sc.nextInt() * sc.nextInt();
		int f = sc.nextInt();
		for(int i=0; i<f; i++){
			sum -= sc.nextInt();
			if(sum < 0){
				sum = -1;
				break;
			}
		}
		System.out.println(sum);
		}
}