In each question, the first and the last sentences of the passage are numbered S1 and S6 respectively. The rest of the passage is split into four parts. These four sentences are jumbled. Read the sentences and identify their correct and logical order.

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S1: The houses in the Indus Valley were built of baked bricks.
P: This staircase sometimes continued upwards on to the roof.
Q: Access to the upstairs rooms was by a narrow stone staircase at the back of the house.
R: The drains were incorporated in the walls.
S: The houses had bathrooms and water closets, rubbish chutes and excellent drainage systems.
S6: They led outside into covered sewers which ran down the side of the streets.
The Proper sequence should be: