PROGRAMMING:Sum up
Find the sum of [1, n].
###Input format:
Enter a positive integer n.
###Output format:
Output the sum of 1 + 2 +... + n.
###Input example:
Here is a set of inputs. For example:
```in
three
```
###Output example:
The corresponding output is given here. For example:
```out
six
```
answer：If there is no answer, please comment
###Input format:
Enter a positive integer n.
###Output format:
Output the sum of 1 + 2 +... + n.
###Input example:
Here is a set of inputs. For example:
```in
three
```
###Output example:
The corresponding output is given here. For example:
```out
six
```
answer：If there is no answer, please comment