import java.util.Scanner;
import java.util.Random;
public class Main {
    public static void main(String[] args) {
        Scanner sss = new Scanner(System.in);
        int ia = sss.nextInt();
        int ib = sss.nextInt();
        long oa = (long) 0;
        for(long i=(long) ia;i<=ib;i+=2){
            oa+=i;
        }
        System.out.println(oa);
    }
}