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.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.*;
import java.io.IOException;

public class Main {

    public static void main(String[] args) {
        OutputStream outputStream = System.out;
        myScanner in = new myScanner();
        PrintWriter out = new PrintWriter(outputStream);
        Solution solver = new Solution();
        solver.solve(1, in, out);
        out.close();
    }


    static class Solution {

        public void solve(int testNumber, myScanner in, PrintWriter out) {
            long n = in.nextLong();

            if(n % 2 == 0) {
                out.println(n / 2);
            } else {
                out.println(-(n/2) - 1);
            }
        }
    }

    static class myScanner {
        BufferedReader br;
        StringTokenizer st;

        public myScanner() {
            br = new BufferedReader(new InputStreamReader(System.in), 32000);
        }

        boolean ready() throws IOException {
            return br.ready();
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }


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

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

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return str;
        }
    }
}
