using System;

namespace IOLibrary
{
    class Program
    {
        static void Main(string[] args)
        {
            var r = new Reader();
            var K = r.nextInt();
            var N = r.nextInt();
            var F = r.nextInt();
            var beans = K * N;
            for(var i = 0; i < F; ++i )
            {
                var An = r.nextInt();
                beans -= An;
            }
            beans = beans < 0 ? -1 : beans;
            Console.WriteLine(beans);
        }
    }

    class Reader
    {
        string[] s;
        int i;
        char[] cs = new char[] { ' ' };

        public Reader()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if ( i < s.Length )
                return s[i++];
            var input = Console.ReadLine();
            while ( input == "" ) input = Console.ReadLine();
            s = input.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            i = 0;
            return next();
        }

        public int nextInt()
        {
            return int.Parse(next());
        }

    }

}