open System

let ri () = stdin.ReadLine() |> int
let ria () = stdin.ReadLine().Split() |> Array.map int

type Sol() =
    member this.Solve() = 
        
        let [|N;K|] = ria ()
        let A = ria ()
        
        let B = Array.sort A
        let mutable ans = 0
        
        for i in 0..(N-1) do
            let n = Array.findIndex (fun x -> x = B.[i]) A
            if ( (n - i) % K ) = 0 then (
                for j in 0..( (n-i)/K - 1 ) do
                (
                    let tmp = A.[(n - j*K)];
                    A.[(n - j*K)] <- A.[(n - j*K) - K];
                    A.[(n - j*K) - K] <- tmp;
                    ans <- ans + 1
                )
            ) else ans <- -1000000000
        
        
        if ans < 0 then ans <- -1
        printfn "%d" ans
        
let mySol = new Sol()
mySol.Solve()