Simple Value Calculator

Hello , 

I need a simple MT4 indicator that help me calculate the average value of the top and bottom , follow formula ( A + B ) / 2 . 

Step 1 : I click a point A on chart 

Step 2 : I click a point B on chart 

Step 3 : It will calculate ( A + B ) / 2 and show final value C on chart with horizontal line .

Please see an example below for more details and how it should work .