Count Strictly Increasing Pairs

Source: Phước Hưng OJ

Version: Phuoc Hung OJ Extended

Problem Statement

Given a nonnegative integer $n$, count integer pairs $(i,j)$ satisfying $1\le i<j\le n$. The answer is zero when $n$ is 0 or 1.

Input

One line contains integer $n$.

Output

Print one integer.

Subtasks

• Subtask 1 (20%): $0\le n\le 20$.

• Subtask 2 (30%): $0\le n\le 200$.

• Subtask 3 (50%): $0\le n\le 1000$.

Examples

Example 1

Input:

4

Output:

6

Explanation:

There are 4 candidates, so the number of i<j pairs is 4×(4-1)/2 = 6.

Example 2

Input:

0

Output:

0

Explanation:

There are 0 candidates, so the number of i<j pairs is 0×(0-1)/2 = 0.