using System;
using System.Linq;

namespace No143
{
    class Program
    {
        static void Main(string[] args)
        {
            var KNF = Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToArray();
            var As = Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToArray();

            var K = KNF[0];
            var N = KNF[1];
            var F = KNF[2];

            var totalM = K * N;
            var totalA = As.Sum();

            var result = totalM - totalA;
            if (result < 0) result = -1;

            Console.Write(result);
        }
    }
}