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Use this:

return sum(N(d,i) for i in range(1,n))

Your syntax is for symbolic sums only.

Use this:

return sum(N(d,i) for i in range(1,n))
range(1,n))

Use this:

return sum(N(d,i) for i in range(1,n))

Your syntax is for symbolic sums only.only. And remove your first line.