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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.io.Reader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;

public class R277_A {

	public R277_A() 
	{
		Scanner s = new Scanner();
		

		long l = s.nextLong();
		
		if (l % 2 == 0)
			System.out.println(l / 2);
		else
			System.out.println(-l/2 - 1);
	}
	
	
	
	
	public static void main(String[] args) {
		new R277_A();
	}

	public static class Scanner {
		BufferedReader br;
		StringTokenizer st;

		public Scanner(Reader in) {
			br = new BufferedReader(in);
		}

		public Scanner() {
			this(new InputStreamReader(System.in));
		}

		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());
		}

		// Slightly different from java.util.Scanner.nextLine(),
		// which returns any remaining characters in current line,
		// if any.
		String readNextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

		public int[] readNextLineInts(int numberOfInts) {
			int[] ints = new int[numberOfInts];
			for (int i = 0; i < numberOfInts; i++)
				ints[i] = nextInt();
			return ints;
		}
		
		public double[] readNextLineDoubles(int numberOfDoubles) {
			double[] doubles = new double[numberOfDoubles];
			for (int i = 0; i < numberOfDoubles; i++)
				doubles[i] = nextDouble();
			return doubles;
		}
		// --------------------------------------------------------
	}
}