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 credit;


import javax.swing.text.MutableAttributeSet;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.beans.IntrospectionException;
import java.io.*;
import java.net.Inet4Address;
import java.sql.SQLIntegrityConstraintViolationException;
import java.time.temporal.ChronoField;
import java.util.*;
import java.util.List;

public class sas {
    static boolean v[];
    static int ans[];
    int size[];
    static int count=0;
    static int dsu=0;
    static int c=0;
    int min=Integer.MAX_VALUE;
    int max=Integer.MIN_VALUE;
    boolean aBoolean=true;
    PrintWriter printWriter = new PrintWriter(System.out);
    static class Reader
        {
            final private int BUFFER_SIZE = 1 << 16;
            private DataInputStream din;
            private byte[] buffer;
            private int bufferPointer, bytesRead;

            public Reader()
            {
                din = new DataInputStream(System.in);
                buffer = new byte[BUFFER_SIZE];
                bufferPointer = bytesRead = 0;
            }

            public Reader(String file_name) throws IOException
            {
                din = new DataInputStream(new FileInputStream(file_name));
                buffer = new byte[BUFFER_SIZE];
                bufferPointer = bytesRead = 0;
            }

            public String readLine() throws IOException
            {
                byte[] buf = new byte[64]; // line length
                int cnt = 0, c;
                while ((c = read()) != -1)
                {
                    if (c == '\n')
                        break;
                    buf[cnt++] = (byte) c;
                }
                return new String(buf, 0, cnt);
            }

            public int nextInt() throws IOException
            {
                int ret = 0;
                byte c = read();
                while (c <= ' ')
                    c = read();
                boolean neg = (c == '-');
                if (neg)
                    c = read();
                do
                {
                    ret = ret * 10 + c - '0';
                }  while ((c = read()) >= '0' && c <= '9');

                if (neg)
                    return -ret;
                return ret;
            }

            public long nextLong() throws IOException
            {
                long ret = 0;
                byte c = read();
                while (c <= ' ')
                    c = read();
                boolean neg = (c == '-');
                if (neg)
                    c = read();
                do {
                    ret = ret * 10 + c - '0';
                }
                while ((c = read()) >= '0' && c <= '9');
                if (neg)
                    return -ret;
                return ret;
            }

            public double nextDouble() throws IOException
            {
                double ret = 0, div = 1;
                byte c = read();
                while (c <= ' ')
                    c = read();
                boolean neg = (c == '-');
                if (neg)
                    c = read();

                do {
                    ret = ret * 10 + c - '0';
                }
                while ((c = read()) >= '0' && c <= '9');

                if (c == '.')
                {
                    while ((c = read()) >= '0' && c <= '9')
                    {
                        ret += (c - '0') / (div *= 10);
                    }
                }

                if (neg)
                    return -ret;
                return ret;
            }

            private void fillBuffer() throws IOException
            {
                bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
                if (bytesRead == -1)
                    buffer[0] = -1;
            }

            private byte read() throws IOException
            {
                if (bufferPointer == bytesRead)
                    fillBuffer();
                return buffer[bufferPointer++];
            }

            public void close() throws IOException
            {
                if (din == null)
                    return;
                din.close();
            }
        }
        int parent[];
        int rank[];
        public static void main(String[] args) throws IOException {
            sas g = new sas();
            g.go();
        }
        public void go() throws IOException {
            Reader scanner = new Reader();
//            Scanner scanner=new Scanner(System.in);
            int t =1;
            for (int i = 0; i < t; i++) {
                long n=scanner.nextLong();
                if(n%2==0)
                printWriter.println(n/2);
                else
                printWriter.println((n/2)-n);
            }
            printWriter.flush();
        }
        public void parent(int n){
            for (int i = 0; i < n; i++) {
                parent[i]=i;
                rank[i]=1;
                size[i]=1;
            }
        }
        public void union(int i,int j){
            int root1=find(i);
            int root2=find(j);
//            if(root1 != root2) {
//                parent[root2] = root1;
////                sz[a] += sz[b];
//            }
            if(root1==root2){
                return;
            }
            if(rank[root1]>rank[root2]){
                parent[root2]=root1;
                size[root1]+=size[root2];
            }
            else if(rank[root1]<rank[root2]){
                parent[root1]=root2;
                size[root2]+=size[root1];
            }
            else{
                parent[root2]=root1;
                rank[root1]+=1;
                size[root1]+=size[root2];
            }
        }
        public int find(int p){
           if(parent[p]!=p){
               parent[p]=find(parent[p]);
           }
           return parent[p];
//            if(parent[p]==-1){
//                return -1;
//            }
//            else if(parent[p]==p){
//                return p;
//            }
//            else {
//                parent[p]=find(parent[p]);
//                return parent[p];
//            }
        }
        public void make(int p){
            parent[p]=p;
            rank[p]=1;
        }
        Random rand = new Random();
        public void sort(int[] a, int n) {
                for (int i = 0; i < n; i++) {
                   int j = rand.nextInt(i + 1);
                   int tmp = a[i];
                   a[i] = a[j];
                   a[j] = tmp;
                }
                Arrays.sort(a, 0, n);
        }
        public long gcd(long a,long b){
               if(b==0){
                  return a;
               }
               return gcd(b,a%b);
        }
        private void dfs2(ArrayList<Integer>[]arrayList,int j,int count){
            ans[j]=count;
            for(int i:arrayList[j]){
                if(ans[i]==-1){
                    dfs2(arrayList,i,count);
                }
            }
        }
        private void dfs(ArrayList<Integer>[] arrayList, int j) {
            v[j]=true;
            count++;
            for(int i:arrayList[j]){
                if(v[i]==false) {
                    dfs(arrayList, i);
                }
            }
        }
        public long fact(long h){
            long sum=1;
            while(h>=1){
                sum=sum*h;
                h--;
            }
            return sum;
        }
        public long primef(double r){
            long c=0;
            while(r%2==0){
                c++;
                r=r/2;
            }
            for (int i = 3; i <=Math.sqrt(r) ;i+=2) {
                while(r%i==0){
                    c++;
                    r=r/3;
                }
            }
            if(r>2){
                c++;
            }
            return c;
        }
}

class Pair{
    long x;
    long y;
    public Pair(long x,long y){
        this.x=x;
        this.y=y;
    }
}
class Sorting implements Comparator<Pair> {
    public int compare(Pair p1,Pair p2){
        return Long.compare(p1.x,p2.x);
    }

}
