using System;
using System.Collections.Generic;
namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
#if LocalDebug
            var exStdIn = new System.IO.StreamReader("stdin.txt");
            System.Console.SetIn(exStdIn);
#endif
            var Input = GetList();
            var BeanNum = int.Parse(Input[0]) * int.Parse(Input[1]);

            var Input2 = GetList();
            var OldSum = 0;

            for (int i = 0; i < int.Parse(Input[2]); i++)
            {
                OldSum += int.Parse(Input2[i]);
            }

            var Output = (BeanNum >= OldSum) ? (BeanNum - OldSum) : -1;
            Console.WriteLine(Output);

#if LocalDebug
            System.Console.ReadKey();
#endif
        }


        static int GetData()
        {
            return int.Parse(Console.ReadLine());
        }

        static string[] GetList()
        {
            return Console.ReadLine().Split(' ');
        }



    }
}