Conditionally sum a range of columns based on another column in R

I've looked through several other related questions but haven't really found something that meets my case.

I have a column that dictates how many columns will be summed into a new column.

• If DEPENDENCY == INDEP then NET_AGI = IND_AGI
• If DEPENDENCY == DEP then NET_AGI = PRO_AGI + IND_AGI
• Otherwise NET_AGI = PRO_AGI

DEPENDENCY IND_AGI PRO_AGI  NET_AGI <- NET_AGI will be the summed column
INDEP      0049995    -     0049995
DEP        0000500 0090500  0091000
DEP        0009000 0121095  0130950
DEP           -    0375001  0375001
INDEP      0123456    -     0123456
DEP        0012070 1023030  1035100
...

What's the best way to do that?