import java.util.Scanner;


public class Main_yukicoder143 {

    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 sum = k * n;
        for (int i = 0; i < f; i++) {
        	sum -= sc.nextInt();
        }

        if (sum >= 0) {
        	System.out.println(sum);
        } else {
        	System.out.println(-1);
        }
        
        sc.close();
    }
}