Scoring only one or zero in one InputField?

Hello Good old Unity community!

My problem is that I don't know how to score only zero or one point in one InputField. I have loads of inputFields in one scene. If the player answers correct he gets +=1 point and if he answers incorrect he gets -=1 point.

How do I make it so that a player can only score one or zero points. Now the player can erase his answer and get more points than one for one question. I got different code for every InputField. Here is my code for the first InputField:

using UnityEngine; using System.Collections; using UnityEngine.UI;

public class Math1: MonoBehaviour {

 public InputField iField;
 public string myAns;

 public int myScore;

 public void Text_Changed(string newText)
 {
     myAns = iField.text;            

     if (myAns == "15")
     {
         Debug.Log("Correct Answer");
         ScoreManager.score +=1;


     }
     else
     {
         Debug.Log("Incorrect");
         ScoreManager.score -=0;

     }
 }

}

Thank you!!