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.*;
import java.util.*;
import java.nio.charset.StandardCharsets;

// remember to check if n=0, n=1 are special cases 

public class A {
    static Writer wr;

    public static void main(String[] args) throws Exception {
        // long startTime = System.nanoTime();

        // String testString = "";
        // InputStream stream = new ByteArrayInputStream(testString.getBytes(StandardCharsets.UTF_8));
        // Reader in = new Reader(stream);

        Reader in = new Reader();
        BufferedWriter out = new BufferedWriter(new OutputStreamWriter(System.out));
        
        wr = new Writer();

        /* Precomputation */


        // long elapsedTime = System.nanoTime() - startTime;
        // double seconds = (double)elapsedTime / 1000000000.0;
        // wr.writeRedLn(seconds);

        /* Input */
        long M = in.nextLong();

        long ans = 0, l1=0, l2=0;
        if(M%2==1) {
            l1 = -1 + ((M/2))*(-2);
            l2 = 2 + ((M/2)-1)*2;
            ans = ((M/2)+1L)*(-1+l1)/2 + (M/2)*(2+l2)/2;
        }
        else {
            l1 = -1 + ((M/2)-1)*(-2);
            l2 = 2 + ((M/2)-1)*2;
            // wr.writeRedLn(l1 + ", " + l2);
            //this line produced overflow
            // ans = ((M/2))*(-1+l1)/2 + (M/2)*(2+l2)/2;
            ans = (((M))*(-1+l1)/4) + ((M)*(2+l2)/4);
        }

        out.write(ans + "\n");
        out.flush();
    }
}



class Writer {
    public void writeRedLn(Object x) { writeRedLn(x+""); }
    public void writeBlueLn(Object x) { writeBlueLn(x+""); }
    public void writeGreenLn(Object x) { writeGreenLn(x+""); }
    public void writePinkLn(Object x) { writePinkLn(x+""); }
    public void writeRedLn(String x) { System.out.println((char)27 + "[31m" + (char)27 + "[40m" + x + (char)27 + "[0m"); }
    public void writeBlueLn(String x) { System.out.println((char)27 + "[34m" + (char)27 + "[3m" + x + (char)27 + "[0m"); }
    public void writeGreenLn(String x) { System.out.println((char)27 + "[32m" + (char)27 + "[3m" + x + (char)27 + "[0m"); }
    public void writePinkLn(String x) { System.out.println((char)27 + "[30m" + (char)27 + "[45m" + x + (char)27 + "[0m"); }
    public void writeRed(String x) { System.out.print((char)27 + "[31m" + (char)27 + "[40m" + x + (char)27 + "[0m"); }
    public void writeBlue(String x) { System.out.print((char)27 + "[34m" + (char)27 + "[3m" + x + (char)27 + "[0m"); }
    public void writeGreen(String x) { System.out.print((char)27 + "[32m" + (char)27 + "[3m" + x + (char)27 + "[0m"); }
    public void writePink(String x) { System.out.print((char)27 + "[30m" + (char)27 + "[45m" + x + (char)27 + "[0m"); }
}


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(InputStream stream){din=new DataInputStream(stream);buffer=new byte[BUFFER_SIZE];bufferPointer=bytesRead=0;}
    public String readLine()throws IOException{byte[] buf=new byte[1024];int cnt=0,c;
        while((c=read())!=-1){if(c=='\n')break;buf[cnt++]=(byte)c;}return new String(buf,0,cnt);}
    public char nextChar()throws IOException{byte c=read();while(c<=' ')c=read();return (char)c;}
    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();}
}
