open System.Text.RegularExpressions

let getOp = function
    | '+' -> (+)
    | '-' -> (-)
    | '/' -> (/)
    | 'X'
    | 'x'
    | '*' -> (*)
    | _ as op -> failwith (sprintf "Invalid operation: %c" op)

let fragmentPattern = @"(\d+)\s+(\d+)\s+([+-/*xX])"

let (|Regex|_|) pattern equasion =
    let matches = Regex.Match(equasion, pattern)
    if matches.Success then Some(List.tail [ for g in matches.Groups -> g.Value ])
    else None

[<EntryPoint>]
let main argv =
    let equasion = argv |> Array.toSeq |> String.concat " "

    let rec solve equasion =
        match equasion with
        | Regex fragmentPattern [num1; num2; op] ->
            let result = (getOp (char op)) (int num1) (int num2)
            let newEquasion = Regex.Replace(equasion, fragmentPattern, sprintf "%i" result)
            solve newEquasion
        | Regex @"^(\d+)$" [number] ->
            printfn "%s" number
        | _ -> printfn "Error"


    solve equasion
    0 // return an integer exit code