input: Write a function to calculate the factorial of a given number using iteration. The function should take an integer `n` as input and return the factorial of `n`. If the input number is negative, return -1 as it's not defined.
  def factorial(n):
   if n < 0:
    return -1
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact
  assert factorial(5)==120
  assert factorial(0)==1
  assert factorial(-3)==-1
  assert factorial(7)==5040
output: def factorial(n):
   if n < 0:
    return -1
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact