How to draw a rectangle on an image in c# from mouse

Hi
My question is that i am having video stream from my webcam.i am displaying it on my picture box.Now i want user to to click on four locations and i want a rectangle drawn on using those four locations i-e first click will be the starting point so on.
I am a beginner so please excuse if you feel its too simple.I can draw a rectangle using hard coded locations.Following is the code i have in my event handler that has the new frame.

Pen pen = new Pen(Color.Red, 2);
Graphics g = Graphics.FromImage(bitmap);
g.DrawRectangle(pen, 30, 30, 60, 30);

but as you see i have hard coded initial x,y,width,height i want them to be user defined clicks.

regards

Tags: C#, .NET, Graphics, Mouse