A $4-bit$ carry-lookahead adder adds two $4-bit$ numbers. The adder is designed only using $AND, OR, NOT, NAND $ and $NOR$ gates. We assume that all the inputs are available in both complemented and uncompleted forms. The delay of each gate is $one$ time unit if we assume that the carry network has been implemented using two-level $AND-OR$ logic.
Thus, the overall propagation delay of the adder is ______ in terms of time units.