Minimum class cancellations

A yoga studio received proposed class slots for its single room as [start, end] pairs, each occupying the half-open span [start, end), unsorted, possibly overlapping. The studio will cancel some proposals so that the surviving classes never overlap (back-to-back is fine). Return the minimum number of proposals it must cancel. Example: [[9,11],[10,12],[11,13]] requires 1 cancellation.