Need help evaluating two pairs or four cells for empty values

greetings from another world. i need help putting together this excel formula.

i want to test four cells for empty values but this evaluation must be performed in pairs. i am subtracting the values of each pair and then adding the difference. the formula i have works for the first cell pair but i do not understand how i can include a second test for e1/d1. the idea is that i dont want excel to perform the calculation if (b1/c1) or (d1/e1) are blank. any help you can provide would be appreciated. thank you.

=IF(OR(ISBLANK(C1),ISBLANK(B1)),TIME(0,0,0),SUM(E1-D1)+SUM(C1-B1)