Find difference only if 2 columns are present, else blank

I expect only to get a result only if both are present. If either is blank, or both are blank, I expect a blank result. Sample data showing each case. Blank cells are intentionally blank, and RoutePriceChange is the expected output value.

AllChanges table:

RoutePriceChange = AllChanges[LastPlanRouteValue] - AllChanges[FirstPlanRouteValue]

and

RoutePriceChange = 
IF( OR(ISBLANK(AllChanges[LastPlanRouteValue]), ISBLANK(AllChanges[FirstPlanRouteValue])),
    BLANK(),
    AllChanges[LastPlanRouteValue] - AllChanges[FirstPlanRouteValue]
    )

but both sample codes provide a value when FirstPlanRouteValue is present AND LastPlanRouteValue is missing.