construction,formula,covered_units,meaning
Two adjacent 410-unit spans,410 + 410,820,No overlap or intervening gap
Two 410-unit spans with a 7-unit gap,410 + 7 + 410,827,The middle seven is added
Two 410-unit spans sharing 7 units,410 + 410 - 7,813,The shared seven is counted once
Two 417-unit spans sharing 7 units,417 + 417 - 7,827,A different pair of spans produces the same 827 total