Another if then statement

=IF(OR(C5="V1",C5="V2",C5="V3"),B6-B5,B6)

In the above code, B5 is "7:00" while B6 is "11:00". Since "V1" was selected for C5, it should subtract 7:00 from 11:00 and return a value of 4 (hours). If V1 had not been selected, it should just display 11:00 (the value of B6).