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
//package codeforces;
import java.io.*;
import java.util.*;
import java.*;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Random;
import java.lang.*;


 
public class zadacha {
private static BufferedReader in;
private static StringTokenizer tok;
private static PrintWriter out;
final static boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") !=null;
 
public static void init() throws FileNotFoundException{
if(ONLINE_JUDGE){
in = new BufferedReader(new InputStreamReader(System.in));
out = new PrintWriter(System.out);
}
else{
in = new BufferedReader(new FileReader("input.txt"));
out = new PrintWriter("out.txt");
}
}
 
private static String readString() throws IOException {
while (tok == null || !tok.hasMoreTokens()) {
tok = new StringTokenizer(in.readLine());
}
return tok.nextToken();
}
 
private static int readInt() throws IOException {
return Integer.parseInt(readString());
}
private static double readDouble() throws IOException {
return Double.parseDouble(readString());
}
private static long readLong() throws IOException {
return Long.parseLong(readString());
}
private static float readFloat() throws IOException{
    return Float.parseFloat(readString());
}
  
public static void Solve() throws IOException{

	long n = readLong();
	if(n%2 ==0 )
		out.print(n/2);
	else out.print(-(n+1)/2);
}
    

public static void main(String[] args) throws IOException {
init();
Solve();
 
 
in.close();
out.close();
}
}