Alternating Sum

Source: Phước Hưng OJ

Version: Phuoc Hung OJ Extended

Problem Statement

Compute 1-2+3-4+... through term n; the empty sum for n=0 is zero. Use a for loop.

Input

One integer 0<=n<=100000.

Output

The alternating integer sum.

Subtasks

• Subtask 1 (20%): n<=10.
• Subtask 2 (30%): n<=1000.
• Subtask 3 (50%): n<=100000.

Examples

Example 1

Input

4

Output

-2

Explanation

1-2+3-4=-2.

Example 2

Input

0

Output

0

Explanation

No terms.