I want a countdown when unpause the game

tried realtimeSinceStartup but I can not put him start so unpause

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class Pause : MonoBehaviour
 {
     public static bool paused = false;
     public GameObject menu;
     public bool isShowing;
     public Button b1;
     public Button b2;
     public Button b3;
     
     //
     
     public float tempo = 4;
     public float tempoPause;
     public bool inicioTempoPause;
     public float StartTimer;
     public float StartTimerValue;
     public Text contador;
     public GameObject contadorObj;
     public bool isUnPaused;
     
     void Start () {
         
         isShowing = false;
         paused = false;
         contadorObj.SetActive (false);
         isUnPaused = false;
         
     }
     
     void Update()
     {
         
         if (isShowing == true) {
             menu.SetActive (true);
             
         }
         if (isShowing == false) {
             menu.SetActive (false);
             
         }    
 
         int tempoTexto = (int)tempo;
 
         if (isUnPaused == true) {
 
             if (tempo > 0) {
                 contadorObj.SetActive (true);
                 tempo -= Time.deltaTime; 
                 contador.text = "            " + tempoTexto;
             }
             if (tempo < 1) {
                 contador.text = "Go!";
             }
             if (tempo == -1) {
                 contadorObj.SetActive (false);
             }
         }
         
                 //dont work ):
 
             StartTimer = Time.realtimeSinceStartup;
         
             if (inicioTempoPause == true) {
                 tempoPause = Time.realtimeSinceStartup - StartTimer;
             }
     }
     
     public void pausedFunction () {
         paused = !paused;
         
         if (paused == true) {
             Time.timeScale = 0;
             isShowing = true;
             PlayerControl.speed = 0;
         }
         
         if (paused == false) {
             Time.timeScale = 0;
             isShowing = false;
             PlayerControl.speed = 0.05f;
             inicioTempoPause  = true;
 
         
         }
     }
     
     
     
 }