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

namespace yukicoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string input2 = Console.ReadLine();
            string[] KNF = input.Split(' ');
            string[] age = input2.Split(' ');
            int mame = int.Parse(KNF[0]);
            int fukuro = int.Parse(KNF[1]);
            int total = mame * fukuro;
            int numfam = int.Parse(KNF[2]);
            for(int i=0;i<numfam;i++)
            {
                total = total - int.Parse(age[i]);
            }
            if(total<0)
            {
                Console.WriteLine("-1");
            }
            else
            {
                Console.WriteLine(total);
            }
        }

    }
}