Factor out 2000 into all of the possible numbers that can be multiplied together in order to get 2000.
1, 2, 4, 5, 8, 16, 25, 40, 50, 80, 125, 250, 400, 500, 1000, 2000
Remove all of those that aren't perfect squares.
4, 16, 400
4 = 2^2
16 = 4^2
400 = 20^2
I think we have a clear winner. 400 is your answer.