$M,$N         final array element (should be $M)
(14,11) ==> final=13
(14,13) ==> final=13

(29,11) ==> final=28
(29,13) ==> final=28
(29,22) ==> final=28
(29,26) ==> final=28

(59,11) ==> final=58
(59,13) ==> final=58
(59,22) ==> final=58
(59,26) ==> final=58
(59,44) ==> final=58
(59,52) ==> final=58
(59,55) ==> final=58

(100,49) ==> final=99
(100,98) ==> final=99
(100,99) ==> final=99