proc expression {curr: Int, input: [Int], ret: Int}
	~ (operator expression expression)
	if input.(curr) = '('
		curr += 01
		var op := input.(curr)
		curr += 01
		var a := 0
		var b := 0
		do expression {curr, input, a}
		do expression {curr, input, b}
		
		if op = 043
			ret += a + b
		else if op = 045
			ret += a - b
		else if op = 042
			ret += a * b
		else if op = 047
			ret += a / b
		else
			error("unrecognized op")
		fi
		fi
		fi
		fi
		
		undo expression {curr, input, b}
		undo expression {curr, input, a}
		drop b := 0
		drop a := 0
		curr -= 01
		drop op := input.(curr)
		
		curr += 03
	~ number
	else if input.(curr) >= 048 && input.(curr) < 058
		ret += input.(curr) - 048
	~ error
	else
		error("neither a group nor a number")
	fi
	fi input.(curr) = ')'
	curr += 01
return

proc main
	var curr := 0
	var input := "(/33)"
	var result := 0
	do expression {curr, input, result}
	do show {result}
	drop result := 0
	drop input := 0
	drop curr := 05
return