I want to avoid String and special characters in textflied. only use numeric key in my project

Question

let characterset = CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@!#$%^&*")

@IBAction func actionManiusAns(sender:UIButton) { if txtFldOne.text == "" && txtFldTwo.text == "" && txtFldThree.text == ""{ alert(Title: "No Input", Message: "Kindly Enter SomeInput") }

    else if txtFldOne.text == "" && txtFldTwo.text != "" && txtFldThree.text != ""{
        alert(Title: "No Input In First Block", Message: "Kindly Enter Some Input In First Block")
    }
    else if txtFldOne.text == "\(characterset)" && txtFldTwo.text == "\(characterset)" &&  txtFldThree.text == "\(characterset)" {
        alert(Title: "Use Numbers", Message: "Kindly Enter Numberic Inputs")
    }
    
    else if txtFldOne.text != "" && txtFldTwo.text == "" &&  txtFldThree.text != "" {
        alert(Title: "No Input In Second Block", Message: "Kindly Enter Some Input In Second Block")
    }
    else if txtFldOne.text != "" && txtFldTwo.text == "" &&  txtFldThree.text == "" {
        alert(Title: "No Input In Second Block", Message: "Kindly Enter Some Input In Second Block")
    }
    else if txtFldOne.text != "" && txtFldTwo.text != "" &&  txtFldThree.text == "" {
        alert(Title: "No Input In Third Block", Message: "Kindly Enter Some Input In Third Block")
    }
    else {
        var firstNumber = Int(txtFldOne.text!)
        var secondNumber = Int(txtFldTwo.text!)
        var thirdNumber  = Int(txtFldThree.text!)
       var ans = Int(firstNumber! - secondNumber! - thirdNumber!)
        
        lblDisply.text = "\(ans)" }
    
}