Answer: Maine entered the Union as a free state and Missouri entered the Union as a slave state.
Further details:
The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added as a free state at the same time, to keep the balance of slave and free states equal. It also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state.