using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        string knf = Console.ReadLine();
        string a = Console.ReadLine();

        int K = int.Parse(knf.Split(' ')[0]);
        int N = int.Parse(knf.Split(' ')[1]);
        int F = int.Parse(knf.Split(' ')[2]);

        int beans = K * N;

        int[] aAry = new int[F];
        aAry = a.Split(' ').Select((s) => int.Parse(s)).ToArray();

        foreach (int age in aAry)
            beans -= age;

        if (beans < 0)
            Console.WriteLine(-1);
        else
            Console.WriteLine(beans);
    }
}