left = 1 and right = n (total number of days).left < right, perform the following steps:
a. Calculate the middle day mid as left + (right - left) / 2.
b. Perform BFS to check if a path exists from the top to the bottom on day mid.
c. If a path exists, set left = mid + 1, otherwise set right = mid.left - 1 will be the last day when it is possible to walk from top to bottom.