input: Write a function to calculate the factorial of a given number. The function should return the factorial of the input number. If the input number is less than 0, the function should return -1 to indicate an invalid input.
  def factorial(n):
   if n < 0:
    return -1
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact
  
  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
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact