17
A biquadratic number is a number that is the fourth power of another integer, for example: 3^4 = 3*3*3*3 = 81
Given an integer as input, output the closest biquadratic number.
Here are the first 15 double-squares:
1, 16, 81, 256, 625, 1296, 2401, 4096, 6561, 10000, 14641, 20736, 28561, 38416, ...