致命错误:意外地发现nil，同时打开一个可选的值计算。

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var yourScore: UITextField!
@IBOutlet weak var totalScore: UITextField!
@IBOutlet weak var labelText: UILabel!
@IBAction func buttonPressed(sender: AnyObject) {
    let score1: Int = yourScore.text.toInt()!
    let score2: Int = totalScore.text.toInt()!
    let mistakes = score2 - score1
    let scoreFinal = ((((score2 / 2) - mistakes)*23)/score2)+75
    labelText.text = "\(scoreFinal)"
}
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}


}
import UIKit

class ViewController: UIViewContr