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.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {

	int mod = 1000000007;

	public static int count(String str, int i) {
		return Character.getNumericValue(str.charAt(i));
	}

	static public int num(String s, int i) {
		return Character.getNumericValue(s.charAt(i));
	}

	
	public static void main(String[] args) {
		MyScanner sc = new MyScanner();
		out = new PrintWriter(new BufferedOutputStream(System.out));

		// Start writing your solution here. -------------------------------------
		long n = sc.nextLong();

		long val = (n + 1) / 2;
		out.print(n % 2 == 1 ? -val : val);

		// room cap

		/*
		 * int n = sc.nextInt(); // read input as integer long k = sc.nextLong(); //
		 * read input as long double d = sc.nextDouble(); // read input as double String
		 * str = sc.next(); // read input as String String s = sc.nextLine(); // read
		 * whole line as String
		 * 
		 * int result = 3*n; out.println(result); // print via PrintWriter
		 */

		// Stop writing your solution here. -------------------------------------
		out.close();
	}

	// -----------PrintWriter for faster output---------------------------------
	public static PrintWriter out;

	// -----------MyScanner class for faster input----------
	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(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());
		}

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

	}
	// --------------------------------------------------------
}
