using System;
using System.Linq;

public class Program {
    public static void Main() {
        string[] input = Console.ReadLine().Split(' ');
        int bean = int.Parse(input[0]);
        int pack = int.Parse(input[1]);
        int person = int.Parse(input[2]);
        int[] age = Array.ConvertAll(Console.ReadLine().Split(' '), delegate(string value) {
            return int.Parse(value);
        });
        Console.WriteLine(Math.Max(bean*pack-age.Sum(), -1));
    }
}