open System

let read() = Console.ReadLine().Split() |> Array.map (int)
             
let K,N,F =
  let t = read()
  t.[0],t.[1],t.[2]             
let A = read()

let num = K * N - (A |> Array.sum)

if num >= 0 then num
else             -1
|> printfn "%i"