input: Write a python function to check if a number is a Fibonacci number.
  assert is_fib(0) == True
  assert is_fib(1) == True
  assert is_fib(2) == True
  assert is_fib(3) == True
  assert is_fib(4) == False
  assert is_fib(5) == True
  assert is_fib(6) == False
  assert is_fib(13) == True
output: def is_fib(n):
      def is_perfect_square(x):
          s = int(x**0.5)
          return s*s == x
      
      return is_perfect_square(5*n*n + 4) or is_perfect_square(5*n*n - 4)