open System

let K, N, F = Console.ReadLine().Split(' ')
              |> fun args -> args.[0] |> int, args.[1] |> int, args.[2] |> int
let As = Console.ReadLine().Split(' ')
         |> Array.map (fun a -> a |> int)
         |> Array.toList

let mutable sum =  K * N

for A in As do
  sum <- sum - A

if sum < 0 
  then -1
  else sum
|> Console.WriteLine