google spreadsheet - ARRAYFORMULA to SUM previous row column and current row column -

https://docs.google.com/spreadsheets/d/1wlwqdfwgv90s50ip-bxxbhciyualohj610qfisatcmq/edit?usp=sharing

shared above sheet example. want arrayformula sum previous rows column , current row column.

in f2 entered formula:

=arrayformula(if(row(b2:b)=2, b2, if(len(b2:b)+len(c2:c), mmult(transpose(if(transpose(row(b2:c))>=row(b2:c),if(b2:b<>"", b2:b,0)+(if(c2:c<>"", c2:c,0)*-1), 0)),row(b2:b)^0),))) 

but.. in order formula work have write 'starting' balance in b2. see spreadsheet shared.

i hope helps ?