How do I put a timer in RollerBall?

I want to give the game a time limit to be completed, and if the player doesnt get all the cubes in that window of time, the game ends and its shows a “You Lose” text.
My code looks like this so far:

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

public class PlayerControler : MonoBehaviour {

	public float speed; 
	public Text countText; 
	public Text winText; 
	public Text timerText; 

	private Rigidbody rb;  
	private int count; 
	void Start ()
	{
		rb = GetComponent<Rigidbody> ();  
		count = 0; 
		SetCountText ();
		winText.text = ""; 
	}
		
	void FixedUpdate ()
	{
		float moveHorizontal = Input.GetAxis ("Horizontal");
		float moveVertical = Input.GetAxis ("Vertical"); 

		Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical); 

		rb.AddForce (movement * speed); 
	}

	void OnTriggerEnter(Collider other) 
	{
		if (other.gameObject.CompareTag ("Pick Up"))
		{	
			other.gameObject.SetActive (false); 
			count = count +  1; 
			SetCountText (); 
		}
	}

	void SetCountText ()
	{
		countText.text = "Count: " + count.ToString (); 
		if (count >= 14) 
		{
			winText.text = "You Win!"; 

		}

	}

}

Please check this code.

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

public class PlayerControler : MonoBehaviour
{

    public float speed;
    public Text countText;
    public Text winText;
    public Text timerText;

    private Rigidbody rb;
    private int count;
    private bool gameFinish = false;

    public float gameTimer = 60;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
        count = 0;
        SetCountText();
        winText.text = "";
    }

    void FixedUpdate()
    {
        gameTimer -= Time.deltaTime;

        if(gameFinish)
        { 
        float moveHorizontal = Input.GetAxis("Horizontal");
        float moveVertical = Input.GetAxis("Vertical");

        Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

        rb.AddForce(movement * speed);
        }

        if(gameTimer<0 || count >= 14)
        {
            gameFinish = true;
            SetResultText();
        }

    }

    void OnTriggerEnter(Collider other)
    {
        if (other.gameObject.CompareTag("Pick Up"))
        {
            other.gameObject.SetActive(false);
            count = count + 1;
            SetCountText();
        }
    }

    void SetCountText()
    {
        countText.text = "Count: " + count.ToString();
    }

    void SetResultText()
    {
        if (count >= 14)
        {
            winText.text = "You Win!";
        }
        else
        {
            winText.text = "You Lose!!!!";
        }

    }

}