Count zeros in a row wise and column wise sorted matrix

Given a N x N binary matrix (elements in matrix can be either 1 or 0) where each row and column of the matrix is sorted in ascending order, count number of 0s present in it.
Expected time complexity is O(N).
Examples:
Input: 
[0, 0, 0, 0, 1]
[0, 0, 0, 1, 1]
[0, 1, 1, 1, 1]
[1, 1, 1, 1, 1]
[1, 1, 1, 1, 1]
Output: 8
Input: 
[0, 0]
[0, 0]
Output: 4
Input: 
[1, 1, 1, 1]
[1, 1, 1, 1]
[1, 1, 1, 1]
[1, 1, 1, 1]
Output: 0
http://www.geeksforgeeks.org/count-zeros-in-a-row-wise-and-column-wise-sorted-matrix/