open System
open System.Collections.Generic

[<AutoOpen>]
module hoge = 
    let inline dump e =
    #if DEBUG
        printfn "%A" e; 
    #endif
        e
    let read() = stdin.ReadLine()
    let reads() = read().Split()
    let readToEnd() = stdin.ReadToEnd()
    let lines(s:string) = s.Split([|stdout.NewLine|], StringSplitOptions.RemoveEmptyEntries) 

let (|ODD|EVE|) n = 
    match n%2 with
    | 0 -> EVE
    | _ -> ODD

let n,m = let t = reads() |> Array.map(int) in t.[0],t.[1]

if n<=m then 1
else
    match n with 
    | ODD -> -1
    | EVE ->
        if n/2<=m then 2
        else          -1
|> printfn "%i"