PROGRAMMING:Average
You are going to read a serial of none-negative integers, which ends with a negative number that does not count as one of the data. Your program calculates the average of all the none-negative integers, and prints the average which rounds to two decimal places.
When there's no valid data at all, your program prints out:
`None`
##Input format:
A serial of none-negative integers, ending with a negative number, which is not one of the valide data.
##Output format:
A number rounds to two decimal places, which is the average of the serial.
The printf for this case is:
`printf("%.2f\n", (double)sum/count);`
##Input examples
```in
1 2 3 4 5 6 -1
```
##Output example
```out
three point five zero
```
answer：If there is no answer, please comment
When there's no valid data at all, your program prints out:
`None`
##Input format:
A serial of none-negative integers, ending with a negative number, which is not one of the valide data.
##Output format:
A number rounds to two decimal places, which is the average of the serial.
The printf for this case is:
`printf("%.2f\n", (double)sum/count);`
##Input examples
```in
1 2 3 4 5 6 -1
```
##Output example
```out
three point five zero
```
answer：If there is no answer, please comment