using System;
public class Program
{
    static void Main()
    {
        string[] inputs = Console.ReadLine().Split(' ');
        int k = int.Parse(inputs[0]);
        int n = int.Parse(inputs[1]);
        int f = int.Parse(inputs[2]);
        inputs = Console.ReadLine().Split(' ');
        int sum = 0;
        foreach (int a in Array.ConvertAll(inputs, s => int.Parse(s)))
        {
            sum += a;
        }
        int r = k * n - sum;
        if (r < 0)
        {
            r = -1;
        }
        Console.WriteLine(r);
    }
}