Area of a rectangle when cell is blank

Hello,
I am the beginner to MS-Excel.I need a solution to this problem:
I have to calculate the area of rectangle(Area =Length*breadth),by testing these condition:
1)If length/breadth is blank,then while calculating the area of rectangle,it should prompt the user to"enter the value".
That is if length is blank,then it should give "enter the value of length" or if breadth is blank,then it should give "enter the value of breadth" or if both length n breadth is not blank,area=l*b;































area