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.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

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

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
import java.io.*;
import java.math.BigDecimal;
import java.util.*;

public class Main {

    FastScanner in;
    PrintWriter out;

    class FastScanner {
        BufferedReader reader;
        StringTokenizer tokenizer;

        FastScanner() throws IOException {
            reader = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() throws IOException {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                String line = reader.readLine();
                if (line == null) {
                    throw new EOFException();
                }
                tokenizer = new StringTokenizer(line);
            }
            return tokenizer.nextToken();
        }

        int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        long nextLong() throws IOException{
            return Long.parseLong(next());
        }
    }


    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    void run() throws IOException{
        in =new FastScanner();
        out = new PrintWriter(new OutputStreamWriter(System.out));
        solve();
        out.flush();
    }

    void solve() throws IOException {
        long n=in.nextLong();
        out.print(n%2!=0?-(n+1)/2:n/2);
    }


}