Charge sessions by block

A media platform meters playback in whole aligned blocks of block seconds counted from launch, so the block holding second t is numbered t divided by block, rounded down. Session i plays from starts[i] up to but not including ends[i], both non negative second counts, and is charged for every block it touches even when it touches only a sliver of one. A session whose bounds match touches nothing. Charged separately, the bill totals every session's touched blocks. Folded onto one connection, a block touched by two or more sessions is charged a single time. Sessions arrive unsorted, second counts reach 10^9 and block is at least 1, so walking blocks one at a time is hopeless. Return how many blocks folding would save.