Here is the problem, I have a grid like below,
now i am at the top-left point, and i want to goto bottom right corner point. from a point i can only go to bottom row, or right column, i cannot backtrack. more simply to say that if i am at (r,c) point where r points to row, and c points to column, i can go from (r,c) to (r+1,c) and (r,c+1) only.
Here is a fact, number of row and column can be larger than 100000
Now how many ways to go to the bottom right corner point from the top left corner ?
is there any combinatorial formula ?