I am creating an app that turns a sphere which is the earth when w is pressed to speed it up and s is used to slow it down. However I am unable how to find out how to make the speed increase. Here is my code:

```
using UnityEngine;
using System.Collections;
public class Spin : MonoBehaviour{

public float speed;

void Update ()
{
    transform.Rotate(Vector3.up,speed * Time.deltaTime);
    if (Input.GetKey ("escape")) {
        Application.Quit();
    }
    if (Input.GetKey ("w")) {
        transform.Rotate(Vector3.up,speed + 1);

    }
    if (Input.GetKey ("s")) {
        transform.Rotate(Vector3.up,speed - 1);

    }
}
}

```

Does anybody have any idea how I can increase the speed for how long the button is pressed?

P.S I wish to write in c#