using System;
using System.Collections.Generic;

public class Program
{
  static void Main()
  {
    string data = Console.ReadLine();
    string _age = Console.ReadLine();
    string Mame_ = "";
    string Fukuro_ = "";
    string Family_ = "";
    int i = 0;

    while (data.Substring(i, 1) != " ")
    {
        Mame_ = Mame_ + data.Substring(i, 1);
        i++;
    }
    i++;
    while (data.Substring(i, 1) != " ")
    {
        Fukuro_ = Fukuro_ + data.Substring(i, 1);
        i++;
    }
    Family_ = data.Substring(i + 1, 1);

    int Mame = Convert.ToInt32(Mame_);
    int Fukuro = Convert.ToInt32(Fukuro_);
    int Family = Convert.ToInt32(Family_);

    //豆の総数
    int Mame_Kei = Fukuro * Mame;

    //家族の年齢
    List<int> age = new List<int>();
    string age_ = "";

    for (int a = 0; a < _age.Length; a++)
    {
        if (_age.Substring(a, 1) != " ")
        {
            age_ = age_ + _age.Substring(a, 1);
        }
        else
        {
            age.Add(Convert.ToInt32(age_));
            age_ = "";
        }
    }

    age.Add(Convert.ToInt32(age_));

    for (int c = 0; c < age.Count; c++)
    {
        Mame_Kei = Mame_Kei - age[c];
    }
    if (Mame_Kei < 0)
    {
        Console.WriteLine("-1");
    }
    else
    {
        Console.WriteLine(Mame_Kei.ToString());
    }

  }
}