Adding two cells then dividing by another

I'm trying to create a sheet that will maintain a running average of my score when I'm playing darts.

B2 = Where I input my score
B3 = Displays my average
B4 = +1 just gets added here everytime I click the button.

The part where I try to define B3 as being B2 + B3 / B4 is the problem.

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