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.InputStreamReader;
import java.util.StringTokenizer;
import java.util.*;
/**
 *
 * @author hp
 */
public class Calculatingfunction {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long n = sc.nextLong();
        long ans=0;
		if (n%2 == 0) ans= n/2;
		else ans = (-1)*(n/2 + 1);
		
		// Output
		//System.out.println(fn);
        System.out.println(ans);
    }    
}
class FastReader {    

    BufferedReader br;    
    StringTokenizer st;    
    
    public FastReader() {        
        br = new BufferedReader(new InputStreamReader(System.in));        
    }    
    
    String next() {        
        while (st == null || !st.hasMoreElements()) {            
            try {                
                st = new StringTokenizer(br.readLine());                
            } catch (Exception 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 (Exception e) {            
            e.printStackTrace();            
        }        
        return str;        
    }    
}
