input: Write a function to generate the nth term of the Fibonacci sequence. The function should take an integer n as input and return the nth term of the Fibonacci sequence. The Fibonacci sequence is defined as: F(0) = 0, F(1) = 1, and for n > 1, F(n) = F(n-1) + F(n-2). If n is less than 0, the function should return -1 to indicate an invalid input.
  
  assert fibonacci_seq(0) == 0
  assert fibonacci_seq(1) == 1
  assert fibonacci_seq(5) == 5
  assert fibonacci_seq(10) == 55
  assert fibonacci_seq(-5) == -1
output: def fibonacci_seq(n):
      if n < 0:
          return -1
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b