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