indexes are at both end of the diagonals for example 0,0, and 8,8
adjacent cells on diagonal - for example 0,0 and 1,0 or 2,0 and //1,1
What do the examples mean ?? I don't get it.

we don't need to loop through m to n and compose sequence for each pattern length. Instead we can compose just one time for pattern length n, and when the sequence length grows between m and n, we count it to sum.