import java.util.*;
import java.lang.*;
import java.io.*;

class Ideone {
  public static void main (String[] args) throws java.lang.Exception {
    int l = -1000000000, r = 1000000000;
    long res = 0;
    for (int x=l; x<=r; x+=2) res+= x;
    System.out.println(res);
  }
}