How to tally series of 1's in a single cell?

Hi
I would like to use excel as a sort of 'dynamic' tally chart (for my classroom) so that as pupils periodically enter a series of, say, 1's in a cell, the total number of 1's appears in an adjacent cell. Would appreciate help.
Sprout

=LEN(A1) - LEN(SUBSTITUTE(A1,1,""))

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