input: Write a function to calculate the factorial of a given number. The function should take an integer input and return the factorial as an integer. If the input is less than 0, the function should return -1 to indicate an invalid input.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(1)==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