Sum of the First N Integers

Source: Phước Hưng OJ

Version: Phuoc Hung OJ Extended

Problem Statement

Given a positive integer $n$, compute $1+2+\cdots+n$ using a closed formula without loops.

Input

One line containing integer $n$.

Output

Print $1+2+\cdots+n$.

Subtask

• Subtask 1 — 100 points: $1\le n\le10^9$.

Example

Input

5

Output

15

Explanation

For $n=5$, the sum $1+2+3+4+5$ is $15$. The formula $5(5+1)/2$ gives the same result, so print 15.