PROGRAMMING:1-2/3+3/5-4/7+5/9-6/11+...
Calculate the sum of the first n terms of the series 1-2/3+3/5-4/7+5/9-6/11+...
###Input format:
Enter a positive integer n.
###Output format:
Output the value of the partial sum, retaining three decimal places.
###Input example:
```in
five
```
###Output example:
```out
zero point nine one seven
```
answer：If there is no answer, please comment
###Input format:
Enter a positive integer n.
###Output format:
Output the value of the partial sum, retaining three decimal places.
###Input example:
```in
five
```
###Output example:
```out
zero point nine one seven
```
answer：If there is no answer, please comment