What am I missing when calculating? - ios

Question

I am pretty new with programming but I am currently learning iOS. I have been trying to build a calculator but I have some problems. For instance, when I try to calculate 2 + 2, it shows me 5. Because when I press the second number (in this case 2) it makes it 3.
Can anyone help me to fix and understand the problem?
import UIKit
class ViewController: UIViewController {
var numberOnScreen:Double = 0
var previousNumber:Double = 0
var performingMath = false
var operation = 0
#IBOutlet weak var label: UILabel!
#IBAction func numbers(_ sender: UIButton)
{
if performingMath == true
{
label.text = String(sender.tag)
numberOnScreen = Double(label.text!)!
performingMath = false
}
else
{
label.text = label.text! + String(sender.tag-1)
numberOnScreen = Double(label.text!)!
}
}
#IBAction func buttons(_ sender: UIButton)
{
if label.text != "" && sender.tag != 11 && sender.tag != 16
{
previousNumber = Double(label.text!)!
if sender.tag == 12 {
label.text = "/"
}
else if sender.tag == 13 {
label.text = "*"
}
else if sender.tag == 14 {
label.text = "-"
}
else if sender.tag == 15 {
label.text = "+"
}
operation = sender.tag
performingMath = true
}
else if sender.tag == 16 {
if operation == 12 {
label.text = String(numberOnScreen / previousNumber)
}
else if operation == 13 {
label.text = String(numberOnScreen * previousNumber)
}
else if operation == 14 {
label.text = String(numberOnScreen - previousNumber)
}
else if operation == 15 {
label.text = String(numberOnScreen + previousNumber)
}
}
else if sender.tag == 11 {
label.text = ""
numberOnScreen = 0
previousNumber = 0
operation = 0
}
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
}