how to get exact random number in c# ?

I want only random number between two. If i do Random.Range(20-40); it chooses any number between 20-40. But i want exactly 20 or 40.

Not other numbers like 27 ,29,35 etc. just 20 or 40.

how can i change the xforce += value in exactly 20 and 40 randomly.

public class ballmovement : MonoBehaviour {

public float movespeed;
public float yforce ;  
public float xforce ;
public float dampenforce;

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

// Update is called once per frame
void Update () {

	float h = Input.GetAxis ("Horizontal") * movespeed;

transform.Translate(Vector3.right * h);

	 
	if (Input.GetKey (KeyCode.Space))
	{

**************xforce += 20f ;************
		transform.Rotate (new Vector3(0, 0, 30.0f) * Time.deltaTime);
		yforce = 2500.0f;
	}

int twentyOrForty = 20 + (Random.Range(0,2) * 20);