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

namespace No143Beans
{
    class Program
    {
        static void Main(string[] args)
        {
            //豆、袋、人数の入力
            string[] K = Console.ReadLine().Split(' ');

            int beans = int.Parse(K[0]);
            int pack = int.Parse(K[1]);
            int people = int.Parse(K[2]);

            //年齢の入力
            string[] N = Console.ReadLine().Split(' ');

            //豆の総数
            int allbeans = (beans * pack);

            //年齢の総数
            int[] year = new int[people];

            int years = 0;
            for (int i = 0; i < people; i++)
            {
                year[i] = int.Parse(N[i]);
                years += year[i];

            }

            //豆の数のほうが多い場合
            if (allbeans >= years)
            {
                //表示
                Console.WriteLine(allbeans - years);
            }

            //年齢の数のほうが大きい場合
            else if (years > allbeans)
            {
                //表示
                Console.WriteLine("-1");
            }
        }
    }
}