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

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] buf1 = Console.ReadLine().Split(' ');
            string[] buf2 = Console.ReadLine().Split(' ');
            int K = int.Parse(buf1[0]);
            int N = int.Parse(buf1[1]);
            int F = int.Parse(buf1[2]);
            int AllBeans = 0;
            AllBeans = K * N;
            for(int i = 0;i < F; i++)
            {
                AllBeans -= int.Parse(buf2[i]); 
            }
            if(AllBeans >= 0)
            {
                Console.WriteLine(AllBeans);
            }
            else
            {
                Console.WriteLine(-1);
            }
        }
    }
}