using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string[] s = Console.ReadLine().Split();
        int k = int.Parse(s[0]);
        int n = int.Parse(s[1]);
        int f = int.Parse(s[2]);
        int[] a = new int[f];
        string[] str = Console.ReadLine().Split();
        for(int i=0;i< f; i++)
        {
            a[i] = int.Parse(str[i]);
        }
        
        int bean = k * n;
        int ans = 0;

        if (bean >= a.Sum()) ans = bean - a.Sum();
        else ans = -1;

        Console.WriteLine(ans);
    }
}