#!/usr/bin/env io

Stack := List clone
Stack evalToken := method(token, 
  if(token == "", return) // a bug in String split?
  num := token asNumber
  if(num, push(num), applyOperator(token))
)
Stack top := method(at(count - 1))
Stack applyOperator := method(op,
  if(Number hasSlot(op),
    if(count < 2, Error raise("pop", "Stack underflow"))
    num2 := pop; num1 := pop
    push(num1 perform(op, num2)),
    Error raise("operator", "Unknown operator: " + op)
  )
)

while(line := File standardInput readLine,
  tokens := line split(" ", "\t")
  try(
    tokens foreach(i, token, Stack evalToken(token))
    if(Stack top, write(Stack top, "\n"))
  ) catch(Exception, e, 
    write(e description, "\n")
  )
)