Chain of Pairs

Given a N * 2 array A where (A[i][0], A[i][1]) represents the i^th pair.

In every pair, the first number is always smaller than the second number.

A pair (c, d) can follow another pair (a, b) if b < c , similarly in this way a chain of pairs can be formed.

Find the length of the longest chain subsequence which can be formed from a given set of pairs.

Note: A subsequence of a given sequence is a sequence that can be derived from the given sequence by deleting some or no elements without changing the order of the remaining elements.

1 <= N <= 10³

1 <= A[i][0] < A[i][1] <= 10⁴

First and only argument is an 2D integer array A of size N * 2 representing the N pairs.

Return a single integer denoting the length of longest chain subsequence of pairs possible under given constraint.

Input 1:

 A = [  [5, 24]
        [39, 60]
        [15, 28]
        [27, 40]
        [50, 90]
     ]

Input 2:

 
A = [   [10, 20]
        [1, 2]
     ]

Output 1:

 3

Output 2:

 1

Explanation 1:

 Longest chain that can be formed is of length 3, and the chain is [ [5, 24], [27, 40], [50, 90] ]

Explanation 2:

 Longest chain that can be formed is of length 1, and the chain is [ [1, 2] ] or [ [10, 20] ]