Can't get check boxes to do what I need

I would like to create a checkbox to do some work for me in an excel spreadsheet. I am a novice and have spent many hours trying to figure this all out.
So here is what I am trying to do:
Place a checkbox in A1
If the check box in A1 is checked it copies the value of B1 and inserts it into C2
If the checkbox in A1 is unchecked it removes the value of B1 that was inserted in C2 above and leaves C2 blank
Place a checkbox in A3
If the checkbox in A3 is checked it copies the value of B3 and inserts it into C2
If the checkbox in A3 is unchecked it removes the value of B3 that was inserted in C2 and leaves C2 blank
If A1 is checked, A3 can't be checked at the same time
If A3 is checked, A1 can't be checked at the same time
Both A1 and A3 can be unchecked however and C2 is left blank.
I have tried recording macros but they don't seem to work properly. Thanks for your help.

Insert a checkbox, select it and select View Code.
Try this code:

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I am using excel in mac. I tried doing "assign a macro" to the check box but the code above didn't work. It hangs on the the second line? Not sure why I am making this so difficult. I was able to do this easily using Numbers for mac but need to do the same sheet in excel to be able to distribute it. When I do a simple conversion the check boxes change and then don't behave the same in Excel as numbers. I know Excel is more powerful but that seems to make it more difficult.
Thanks for the help.