         let inputText = sender.titleLabel!.text!
         var currentText = resultLabel.text!
         
-        if currentText == "0" || (previousValue != nil && currentValue == nil) {
+        if currentText == zero || (previousValue != nil && currentValue == nil) {
             currentText = inputText
         } else {
             currentText = currentText + inputText

    }
    
    @IBAction func onNegativeTapped() {
        if var currentText = resultLabel.text where currentText != "0" {
            let firstCharacter = currentText[currentText.startIndex]
            
            if firstCharacter == "-" {
                currentText.remove(at: currentText.startIndex)
                resultLabel.text = currentText
            } else {
                resultLabel.text = "-" + currentText
            }
        }
    }
    
    @IBAction func onOperationTapped(_ sender: CalculatorButton) {
        previousValue = Double(resultLabel.text!)!
        currentValue = nil
        sender.isHighlighted = true
    }
    
    @IBAction func onClearTapped() {
        resultLabel.text = "0"
        currentValue = nil
    }
    

        var currentText = resultLabel.text!
        
        if currentText.characters.count == 1 {
            if currentText != zero {
                resultLabel.text = zero
            }
        } else {
            currentText.remove(at: currentText.index(before: currentText.endIndex))
