want to find the distance between two cities in Asp.net App

I developed an application in which i used image map control(with many hot spots ) and when i first time click on the one city then on the level the name of city comes and next clicked city name comes in another level but  i want to calculate the distance between these two selected cities. I try many method offline which uses many mathematical formulas but i didn't get the adjact distance so help me to find the adjact distance. 
 Is there any method to get the distance from google map....
i also attached my site UI below......