An emergency network places directional receivers along a ridge. A receiver covers every point in its signal corridor, represented by an interval [left, right]. A corridor is visible unless one other corridor covers it completely. Two identical corridors are considered indistinguishable, so neither is visible when both are present.

Return the number of visible corridors. Corridors that are completely covered by another corridor do not count. Touching at an endpoint is allowed: [1, 3] and [3, 7] each retain their own visible points.