input: Write a function to calculate the factorial of a given number using an iterative approach. If the input number is negative, return -1 to indicate an error. If the input number is 0, return 1 as the factorial of 0 is 1.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(-1)==-1
  assert factorial(5)==120
  assert factorial(3)==6
output: def factorial(n):
      if n < 0:
          return -1
      result = 1
      for i in range(1, n + 1):
          result *= i
      return result