Codeforces Round #277 (Div. 2)---A. Calculating Function （規律）

Codeforces Round #277 (Div. 2)---A. Calculating Function （規律）

Calculating Function
time limit per test 1 second memory limit per test 256 megabytes input standard input output standard output

For a positive integer n let's define a function f:

f(n)?=??-?1?+?2?-?3?+?..?+?(?-?1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1?≤?n?≤?1015).

Output

Print f(n) in a single line.

Sample test(s) input

4

output

2

input

5

output

-3

Note

f(4)?=??-?1?+?2?-?3?+?4?=?2

f(5)?=??-?1?+?2?-?3?+?4?-?5?=??-?3

解題思路：大水題一枚，直接找規律。n%2==0時，f = n/2; 否則，f = -(n+1)/2.

AC代碼：

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