CF 277 div2 A. Calculating Function

A. 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

# include<cstdio>
# include<iostream>

using namespace std;

int main(void)
{
    long long n;
    while(scanf("%lld",&n)!=EOF)
    {
        if ( n%2 == 0 )
            printf("%lld\n",n/2);
        else
            printf("%lld\n",n/2-n);
    }

    return 0;
}