summing up

hi m8's

i have a column which holds numbers and i have a another column which holds words,
for E.g:
A======================B
1 Top Up-----------------[1] 1000
2 New--------------------[2] 100
3 Top up-----------------[3] 10

\1

i want rows which are Top Up (B1 & B2) to be auto Calculated, how?

thanks and regards