St Andrews Online Judge

Even Sum

By Kay Akashi

Time: 1000 ms
Memory: 256000 kB

The code judging system is only available during contests. Check out the github repo for test cases and solutions.

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Problem Statement

Given an even positive number nn, output the sum of even positive numbers smaller than or equal to nn.

Constraints

0n1050 \leq n \leq 10^{5}.

Input

The input consists of one number nn.

Output

Output the answer.

Examples

Input

2

Output

2

Explanation

The even positive numbers smaller than or equal to 22 is 22 itself, only. Hence, the answer is 22.

Input

10

Output

30

Explanation

The even positive numbers smaller than or equal to 1010 are 22, 44, 66, 88, and 1010. Hence, the answer is 2+4+6+8+10=302 + 4 + 6 + 8 + 10 = 30.