Need Help with a formula.

I am needing help.  I don't know what I'm missing.  I am trying to add amounts from different worksheets.  I've figured out part of it but now it's not adding all the numbers I need.  

I have three worksheets and two amounts in each worksheet that I need added to come up with a total on the fourth worksheet which is my last worksheet.  It's only totaling one amount in each worksheet.

My Worksheets are titled APRIL.2017, MAY.2017 and JUNE.2017.  

This is the formula that I have right now:    =SUM(APRIL.2017!F2,F42,MAY.2017!F2,F42,JUNE.2017!F2,F43)

Its not calculating the F42, F42 and F43.

What am i missing?