Unity Countdown timer in C#

I am creating a level of puzzle game in which user have to connect the red wire and and green wire.
In wires at corner there are various shapes Like L-Shape , T-Shape and I-Shape pipe. I am rotaing the pipe of green wire and if green wire connection success then it should start timer say 20 seconds then user get only 20 seconds to connect red wire otherwise gameover.
So please help me in timer.

Assume that Green wire connection is succesful.

GameMonitor.cs

using UnityEngine;
using System.Collections;

public class GameMonitor : MonoBehaviour {

	GTShape1 GTS1;

	// Use this for initialization
	void Start () {
	
	}

	void Awake()
	{
		GTS1 = GameObject.FindGameObjectWithTag ("GTShape1").GetComponent<GTShape1> ();


		}
	// Update is called once per frame
	void Update () {
		if (GTS1.success) {

			print ("Green Wire Connected");
		}
	}
}

This should do it!

using UnityEngine;
using System.Collections;

public class GameMonitor : MonoBehaviour {
	
	GTShape1 GTS1;
	
	// Use this for initialization
	void Start () {
		
	}
	
	void Awake()
	{
		GTS1 = GameObject.FindGameObjectWithTag ("GTShape1").GetComponent<GTShape1> ();
		
		
	}
	// Update is called once per frame
	void Update () {
		if (GTS1.success) {

			print ("Green Wire Connected");
			StartCoroutine(Timer());
			print ("Time is Up!");
		}
	}

	IEnumerator Timer ()
	{
		yield return new WaitForSeconds(20.0f);
	}
}

For more help: http://unity3d.com/learn/tutorials/modules/intermediate/scripting/coroutines