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

namespace yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            string[] t = s.Split(' ');
            int mame = int.Parse(t[0]) * int.Parse(t[1]);
            int[] ages = new int[int.Parse(t[2])];

            s = Console.ReadLine();
            string[] k = s.Split(' ');
            for(int i = 0; i < k.Length; i++)
            {
                mame = mame - int.Parse(k[i]);
            }

            if(0 <= mame)
            {
                Console.WriteLine("{0}", mame);
            }
            else
            {
                Console.WriteLine("{0}", "-1");
            }
            
            //Console.ReadLine();

        }
    }
}