using System;
using System.Collections.Generic;
using System.Text;

public class Program
{
    public void Proc(){
        Reader.IsDebug = false;
        int[] inpt = Reader.GetInt();
        
        int mamePerPack = inpt[0];
        int packCount = inpt[1];
        int kazoku = inpt[2];
        
        int[] nenrei = Reader.GetInt();
        
        int ans = mamePerPack * packCount;
        
        foreach (int eat in nenrei)
        {
            ans-=eat;
        }
        if(ans >=0) {
            Console.WriteLine(ans);
        } else
        {
            Console.WriteLine(-1);
        }
    }

    public static void Main(string[] args)
    {
        Program prg = new Program();
        prg.Proc();
    }
}

class Reader
{
    public static bool IsDebug = true;
    
    private static System.IO.StringReader sr;
    
    public static string ReadLine() {
        if(IsDebug) {
            if(sr == null) {
                sr = new System.IO.StringReader(initStr.Trim());
            }
            return sr.ReadLine();
        } else {
            return Console.ReadLine();
        }
    }
    
    public static int[] GetInt(char delimiter = ' ') {
        string[] inpt = ReadLine().Split(delimiter);
        int[] ret = new int[inpt.Length];
        for(int i=0; i<inpt.Length; i++) {
            ret[i] = int.Parse(inpt[i]);
        }
        return ret;
    }
    
    private static string initStr = @"


10 10 3
10 20 40


    ";
}