import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int k = sc.nextInt();
        int n = sc.nextInt();
        int f = sc.nextInt();
        int[] a = new int[f];
        for(int i = 0; i < f; i++)
            a[i] = sc.nextInt();

        int ans = Integer.MAX_VALUE;
        int beans = k * n;
        for(int ai : a) {
            beans -= ai;
            ans = beans;
            if(beans < 0) {
                ans = -1;
                break;
            }
        }

        System.out.println(ans);
    }

}