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

namespace 競プロ
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            var strs = s.Split(' ');
            int countBean = int.Parse(strs[0]);
            int countBag = int.Parse(strs[1]);
            int countFam = int.Parse(strs[2]);

            int countAllBead = countBean * countBag;

            s = Console.ReadLine();
            strs = s.Split(' ');
            List<int> ageList = new List<int>();
            for (int i = 0; i < countFam; i++)
            {
                countAllBead-= int.Parse(strs[i]);
            }

            var res = countAllBead > 0 ? countAllBead : -1;
            Console.WriteLine(res.ToString());


            Console.ReadLine();
        }
    }
}