let split (s:string) = s.Split ' '

let sum0 = split >> Array.map int >> fun (a:int[]) -> a.[0] * a.[1]
let sum1 = split >> Seq.map int >> Seq.sum

let count =
    match (stdin.ReadLine () |> sum0,
           stdin.ReadLine () |> sum1) with
    | (a, b) when a >= b -> a - b
    | _ -> -1

count |> printfn "%d"