let ``No.64 XORフィボナッチ数列1``() =
    let f (s:string) =
        let arr = s.Split(' ') |> Array.map (bigint.Parse)
        let rec fib (a:bigint) b n =
            if n < 0I then a.ToString()
            elif n = 0I then b.ToString()
            else fib b (a^^^b) (n-1I)
        fib arr.[0] arr.[1] (arr.[2]-1I)
    f <| stdin.ReadLine() |> printfn "%s"