import java.util.Scanner;
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
		int sumBeans = a*b;
		int c = sc.nextInt();
		int eatSumBeans = 0;
		int [] familyNumber = new int[c];
		
		for(int i = 0; i<familyNumber.length; i++){
			int d = sc.nextInt();
			eatSumBeans= eatSumBeans+d;
		}
		int result = sumBeans - eatSumBeans;
		if(result <= 0){
			System.out.println(-1);
		}else{
        	System.out.println(result);
		}
    }
}