If formula

This formula is killing me!

I have a column of numbers E3:E12. These are dollar amounts of bills that have been paid.

E13 contains the sum of those numbers.

F3:F12 are cells I put an "X" in when the bill has been paid.

I want G13 to have a formula that will subtract the "E" cell numbers from the E13 cell, but only if there is an "X" in the corresponding "F" cell.

E3:E12 F3:F12
$$$$
$$$$ X
$$$$ X
$$$$
$$$$ X
E:13
(SUM) G13
$$$$ E13 minus any E3:E12 cell with an "X" in the "F" cell
SO, E3 will subtract from E13, if F3 has an "X" in the cell.

I can't figure it out. Maybe that is too much for one cell??

Thanks,
Brandon