import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Setsubun{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int K = sc.nextInt();
        int N = sc.nextInt();
        int num_beans = K*N;

        int F = sc.nextInt();
        int num_eats = 0;
        for(int i = 0; i < F; i++){
            num_eats += sc.nextInt();
        }

        if(num_beans >= num_eats){
            System.out.println(num_beans - num_eats);
        }else{
            System.out.println("-1");
        }
    }
}