Given 2 numbers X and Y find value of max (P&Q) such thatX<=P<Q<=Y

Question asked in delhivery interview

Pls help

Constraints on X and Y?

10^18 are constraints

answer would be `max((Y-1)&Y,(Y-1)&(Y-2))`

If Y is odd then the answer is (Y-1)&Y

if Y is even then the answer is (Y-1)&(Y-2)