PROGRAMMING:Find the sum of 1 + (1 + 2) + (1 + 2 + 3) +... + (1 + 2 + 3 +... + n)

Find the sum of 1 + (1 + 2) + (1 + 2 + 3) +... + (1 + 2 + 3 +... + n)
###Input format:
Enter an integer.
###Output format:
Output the sum.
###Input example:
Here is a set of inputs. For example:
```in
three
```
###Output example:
The corresponding output is given here. For example:
```out
sum = 10
```







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