module NPI
open System.Text.RegularExpressions
type tok = Int of int |Operator of (int -> int -> int)
let parse_op = function
    "-" -> Operator (-) | "/" -> Operator (/) | "*" -> Operator (*) | _ -> Operator (+)
let parser (expr : string) =
    Regex.Split(expr, "(\s|\t|\+|\-|\/|\*)")
    |> Array.toList
    |> List.filter (fun x -> x <> "" && x <> " ")
    |> List.map (fun x -> match x with |"+"|"-"|"*"|"/" -> parse_op x |integer -> Int (int x))
let exec (expr: string) : int =
    let rec exec stack (expr : list<tok>) =
        match expr with
        | Int x :: rest -> exec (x :: stack) rest
        | Operator formulas :: rest ->
            let (a::b::xs) = stack
            exec (formulas a b :: xs) rest
        | [] -> List.head (List.rev stack)
        | _ :: rest -> exec stack rest
    and exec_expr = exec []
    expr
    |> parser
    |> exec_expr