[ Text problems (other).]

In a seven-story building, house spirits live. The elevator travels between the first and the last floors, stopping at each floor. On each floor, starting from the first, one house spirit entered the elevator, but no one exited. When the thousandth house spirit entered the elevator, the elevator stopped. On which floor did this happen?
Let's find out how many house elves ended up in the elevator for a trip from the first to the seventh floor and back, until the elevator returned to the first floor. One house elf entered on the first and seventh floors, and two house elves entered on all other floors. Thus, 12 house elves enter the elevator in one trip.

$1000=83 \cdot 12+4$. This means that after 83 trips, 4 more house elves will be able to enter the elevator: on the first, second, third, and fourth floors.

## Answer

On the fourth floor