Factorial is a mathematical function denoted by '!'. It is defined as

n factorial = n!= 1*2*3...*(n-1)*n

In this question, you have to make a function that will take an integer as input, and return the factorial of that integer if that integer is greater than or equal to zero and return -1 if the number is less than zero or negative.

Note: the function doesn't return print the factorial but returns it.

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Input:

An integer n

Output:

The function returns n! if n is greater than or equal to 0.

0! = 1

and the function returns -1 if the number is less than 0.

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Sample input:

3

Sample output:

6

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Sample input:

-4

Sample output:

-1

**Solution**

```
#take the input here
number=int(input())
#the function definition starts here
def factorial(n):
#write the funtion here that finds and RETURNS factorial of next
if n<0:
return -1
elif n==0:
return 1
else:
f=1
for i in range(1,n+1):
f = f*i
return f
#function definition ends here
#do not alter the code typed below
k=factorial(number)
print(k)
```