input: Write a python function to return the nth Fibonacci number.
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(3) == 2
  assert fibonacci(4) == 3
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
output: def fibonacci(n):
      if n == 1 or n == 2:
          return 1
      a, b = 1, 1
      for _ in range(3, n + 1):
          a, b = b, a + b
      return b