(Qlick SenseCloud) Cumulative total aggregation (set expression)

[#1]

Hello all,

I stuck with simple task:

Please help me with formula, how I can get cumulative total summarizing current period result and previous period results.

I suppose it should be using set expression, like:

((sum({<Week = {"<=$(Week)"}"}>}(Income-Outcome)),

but it doesn't work (((

Thank you in advance!

Regards,

Tima