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

namespace ConsoleApplication2
{
    class Program
    {
        class MameBukuro
        {
            public int Num { get; set; }
            public MameBukuro(int num)
            {
                Num = num;
            }
        }

        class Human
        {
            public int Age { get; set; }
            public Human(int age)
            {
                Age = age;
            }
        }

        static void Main(string[] args)
        {
            var input = Console.ReadLine().Split();
            var mameNum = int.Parse(input[0]);
            var mameBukuroNum = int.Parse(input[1]);
            var familyNum = int.Parse(input[2]);
            var familyAges = Console.ReadLine().Split();

            var mameBukuroList = new List<MameBukuro>();
            for(int i=0; i<mameBukuroNum; i++)
            {
                mameBukuroList.Add(new MameBukuro(mameNum));
            }

            var family = new List<Human>();
            for(int i=0; i<familyNum; i++)
            {
                family.Add(new Human(int.Parse(familyAges[i])));
            }

            var mameSum = mameBukuroList.Sum(x => x.Num);
            var familySum = family.Sum(x => x.Age);
            if (mameSum >= familySum)
            {
                Console.WriteLine(mameSum - familySum);
            }
            else
            {
                Console.WriteLine("-1");
            }

        }
    }
}