https://discuss.codechef.com/questions/4219/post-editorial

4th line of explanation-----

In particular, this means that every shortest path will pass through exactly one point on the diagonal consisting of points of the form (i, N-i) (call this Diagonal A), and exactly one point on the diagonal consisting of points of the form (W-i, H+i-N)

What the heck is ’ i ’ in Here ??

Plz specify.