input: Write a function to calculate the factorial of a given number using a loop. The function should take an integer as input and return the factorial of that number. If the input number is less than 0, return -1.
  
  assert factorial(7) == 5040
  assert factorial(5) == 120
  assert factorial(0) == 1
  assert factorial(-3) == -1
output: def factorial(n):
      if n < 0:
          return -1
      result = 1
      for i in range(1, n + 1):
          result *= i
      return result