using System;
namespace yukicoder
{
    class Program
    {

        static void Main(string[] args)
        {

            var S = Console.ReadLine();
            //int N = int.Parse(Console.ReadLine());
            //int F = int.Parse(Console.ReadLine());
            int mame ,i , j , K , N , F;
            int sum = 0;
            int k = 0;
            //1行目入力部分
            var x = S.Split(" ");
            K = int.Parse(x[0]);
            N = int.Parse(x[1]);
            F = int.Parse(x[2]);
            mame = K * N;

            int[] ints = new int[F*2];
            //2行目入力部分
            var s = Console.ReadLine();
            var z= s.Split(" ");

            for (j = 0; j <= z.Length - 1; j++)
            {
                ints[k] = int.Parse(z[j]); //配列0(1,2)に文字列1(3,5)番目を入れる
                sum += ints[j];//sumに豆の合計
                k++;
            }
            if(mame - sum >= 0)
            {
                j = mame - sum;
                Console.WriteLine(j);
            }
            else
            {
                Console.WriteLine("-1");
            }
        }
    }
}