I have to enter the following formula into excel

ABM x AS x ABFy+m,s + AMC x (1-MCFy+m,s) + APC x (1- PCFy+m,s) + ATA
Ihave worked out the figures for each of the lettered symbols and put them
into my worksheet so my formula reads as follows:

=SUM(H6*H7*H8+H9*(1-H10)+H11*(1-H12)+H13)

However the figure is not coming out anywhere near the same as it works out
on the calculator.

I have tried changing H10 and H12 so that the figure in that cell is already
been calculated (so figure changed from say 2 to -1, it is always a minus
number which I think is what is causing my issue.

Can anyone see where i have gone wrong with this, I have put extra brackets
in but they don't seem to help