Problem 2468. Calculate the h-index

H-index is a powerful tool for quantifying the scientific contribution of a researcher. H-index is defined as follows (source - wikipedia):

"a scholar with an index of h has published h papers each of which has been cited in other papers at least h times".

In this problem, citations of published works of an author will be provided as a vector. Each element of the vector denotes the number of citations of a specific paper of the author. Calculate the h-index.

Example:

Input = [3 3 2 1]; Output = 2