# Problem 213: Flea Circus
A 30×30 grid of squares contains 900 fleas, initially one flea per
square. When a bell is rung, each flea jumps to an adjacent square at
random (usually 4 possibilities, except for fleas on the edge of the
grid or at the corners). What is the expected number of unoccupied
squares after 50 rings of the bell? Give your answer rounded to six
decimal places.