How to put a countdown timer on my roll-a-ball unity tutorial?

I require a countdown timer of about 5 mins and then gameover. Script for C#
Also I need like something to say that if i picked up all the required items = the time stops and game has been won.

 

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1. class Timer :MonoBehaviour{
2. float timeLeft = 300.0f; //its 5 min = 300 sec
   
3. void Update()
4. {
5. timeLeft -=Time.deltaTime;
6. if( timeLeft <0)
7. {
8. GameOver();
9. }
10. }
11. void GameOver()
12. {
13. // Do whatever you want to do here when timer expires..
14. }
15. }

 

What exactly would change this script if I wanted to add 1 second every time I picked up pickup item?