Having problem with a basic guess the number script

using UnityEngine; using System.Collections;

public class NumberWizard : MonoBehaviour {

 int max;
 int min;
 int guess;

 // Use this for initialization
 void Start () {
     
     StartGame ();
 }
 
 void StartGame () {
 
     max = 1000;
     min = 1;
     guess = 500;
 
     
     
     print ("===========================================");
     print ("Welcome to Number Wizard");
     print ("Pick a Number in your head But don't tell me!");
     
     print ("The highest number you can pick is " + max);
     print ("The lowest number you can pick is " + min);
     
     print ("Is the number higher or lower then " + guess);
     print ("UP = higher, DOWN = lower, return = equal");
     
     max = max + 1;
     
     
     
 }
 
 
 
 // Update is called once per frame
 void Update () {
 
     if (Input.GetKeyDown(KeyCode.UpArrow)) 
     
     {
         
         min = guess;
         NextGuess ();
         
         
         
     } 
     
     else if (Input.GetKeyDown(KeyCode.DownArrow)) 
         
     {
                     
         max = guess;
         NextGuess ();
         
         
     } 
     
     else if (Input.GetKeyDown(KeyCode.Return)) 
         
     {
         print ("I won!");
         StartGame ();
     }
 

}

 void NextGuess () {
     
     guess = (max + min) / 2;
     print ("Higher or lower than " + guess);
     print ("UP = higher, DOWN = lower, return = equal");
     
     
 }
 

}

Above is the code ... What i really wanted to do is for it to guess the number ... but as soon as i start pressing up or down arrow like 6 - 7 times , the guess gets stuck in a single number as indicated by the image and result in console below ... dont understand this behavior at the moment ..... Can u plz help me out in this regard ... Thanks