Unknown member: Input.GetMouseButton

Monodevelop is not letting me use GetMouseButton. I want to know if the right mouse button is pressed, basic stuff. But it is launched even when it is not, and if I put a breakpoint on the doSomething(), the GetMoustButton shows global::UnityEngine.Input.GetMouseButton : Unknown member: Input.GetMouseButton.

void Update () {
	if (Input.GetMouseButton(1)) {
       doSomething();
    }
}

You have probably created your own script called Input. That will interfere with the inbuilt Input class as scripts automatically have similarly named classes build for them.