Mex Subsequence

Ridbit is given an array $a_1, a_2, \ldots, a_N$. He needs to find the number of ways to divide the array into contiguous subarrays such that: - Each element of the sequence $a$ belongs to exactly one of the subarrays. - There is an integer $m$ such that the MEX of every subarray is equal to $m$. The MEX of a sequence is the smallest non-negative integer which does not occur in this sequence.