input: Write a function to calculate the factorial of a given number. If the given number is negative, return -1 as it's not possible to have a factorial of a negative number. If the given number is 0, return 1 since 0! is 1.
  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(-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