using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        var input1 = Console.ReadLine();
        var input2 = Console.ReadLine();
        var s_input1 = input1.Split(' ');
        var s_input2 = input2.Split(' ');

        // 1袋あたりの粒
        int K = Convert.ToInt32(s_input1[0]);
        // 袋の数
        int N = Convert.ToInt32(s_input1[1]);
        // 家族の人数
        int F = Convert.ToInt32(s_input1[2]);
        // 各年齢
        var A = new List<int>();

        int mameSum = K * N;

        int ageSum = 0;
        foreach (var str in s_input2)
        {
            ageSum += Convert.ToInt32(str);
        }

        int answer = mameSum - ageSum;
        if (answer < 0)
            answer = -1;

        Console.WriteLine(answer);
    }
}