input: Write a python function to find the nth number in the Fibonacci sequence.
  assert nth_fibo(1) == 1
  assert nth_fibo(2) == 1
  assert nth_fibo(3) == 2
  assert nth_fibo(10) == 55
output: def nth_fibo(n):
      if n <= 0:
          return 0
      elif n == 1 or n == 2:
          return 1
      else:
          a, b = 1, 1
          for _ in range(n - 2):
              a, b = b, a + b
          return b