using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Programming
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] str = Console.ReadLine().Split(' ');
            int K = int.Parse(str[0]);
            int N = int.Parse(str[1]);
            int F = int.Parse(str[2]);
            int[] A = new int[F];

            string[] str2 = Console.ReadLine().Split(' ');
            for (int i = 0; i < F; i++)
            {
                A[i] = int.Parse(str2[i]);
            }

            //------------------------------------

            int needBeen = 0;
            foreach (var item in A)
            {
                needBeen += item;
            }

            int total = K * N - needBeen;

            if (total < 0) total = -1;
            Console.WriteLine(total);
        }
    }
}