input: Write a python function to find the nth Fibonacci number.
  assert find_fibonacci(5) == 5
  assert find_fibonacci(10) == 55
  assert find_fibonacci(15) == 610
output: def find_fibonacci(n):
      if n <= 0:
          return 0
      elif n == 1:
          return 1
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b