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.util.StringTokenizer;

public class CalculatingFunction {
    private static void solve(FastScanner fs) {
        // add code
        long n = fs.nextLong();
        long even = 0, odd = 0;
        if(n%2 == 0)
            even = odd = n/2;
        else {
            even = n / 2;
            odd = even + 1;
        }
        System.out.println((even*(even+1)) - (odd * odd));
    }

    public static void main(String[] args) {
        FastScanner fs = new FastScanner();
        solve(fs);
    }

    static class FastScanner {
        BufferedReader br = null;
        StringTokenizer st;
        
        FastScanner() {
            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;
        }

        long[] readArray(int n) {
            long[] arr = new long[n];
            for(int i=0; i<n; i++)
                arr[i] = nextLong();
            return arr;
        } 
    }
}