-157778763 218978790 976692563 591093088
 -1 -4 1 4
 -999999999 -1000000000 -1 0
 1000000000 1000000000 1000000000 1000000000
 -2 -3 -2 1
 -1000000000 -999999999 1000000000 999999999
 0 -1 0 1
 -999999999 -999999999 999999999 999999999
 -946749893 -687257665 -539044455 -443568671
 -26644507 -867720841 975594569 264730225
 309857887 -687373066 663986893 403321752
 -2 -2 -2 0
 0 -2 0 2
 0 -1 2 1
 -2 -2 2 2
 -999999999 -1000000000 999999999 1000000000
 -330513944 -970064382 500608496 369852884
 0 -3 0 3
 0 0 2 2
 -946749893 -687257666 -539044455 -443568670
 -330513944 -970064383 500608496 369852885
 -1000000000 -1000000000 1000000000 1000000000
 0 -1 2 3
 -999999999 -999999999 -1 -1
 309857887 -687373065 663986893 403321751
 -482406510 -512306895 412844236 -168036049
 0 0 2 0
 0 0 0 0
 -471257905 -685885154 782342299 909511044
 -482406510 -512306894 412844236 -168036050
 -157778763 218978791 976692563 591093087
 -411495869 33834653 -234317741 925065545
 -2 -3 2 3
 -411495869 33834652 -234317741 925065546
 -26644507 -867720842 975594569 264730226
 -537640548 -254017710 62355638 588691834
 -471257905 -685885153 782342299 909511043
 -1 -3 1 3
 1000000000 999999999 1000000000 999999999
 1 0 5 6
 -537640548 -254017711 62355638 588691835

211076501291102387 14 500000000000000000 1 3 2000000000000000000 2 1999999998000000001 49676664342971903 567493356068872580 193123336596257367 2 3 5 13 2000000000000000000 556817654843544374 4 5 49676664750677342 556817655674666815 2000000002000000001 8 499999999000000001 193123336242128360 154104366473536355 2 1 999994501061310398 154104365578285608 211076500156631060 78953311064369599 18 78953311241547728 567493357071111657 252811256874252458 999994499807710193 11 1 18 252811257474248645
import java.io.*;
import java.util.*;

public class E {

    static class InputReader {
        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }
    }

    static InputReader r = new InputReader(System.in);
    static PrintWriter pw = new PrintWriter(System.out);

    public static void main(String[] args) {
        long x1 = r.nextLong();
        long y1 = r.nextLong();
        long x2 = r.nextLong();
        long y2 = r.nextLong();
        long x1o, x1e, y1o, y1e, x2o, x2e, y2o, y2e;

        if(x1 % 2 == 0){
            x1o = x1 + 1;
            x1e = x1;
        } else{
            x1o = x1;
            x1e = x1 + 1;
        }

        if(y1 % 2 == 0){
            y1o = y1 + 1;
            y1e = y1;
        } else{
            y1o = y1;
            y1e = y1 + 1;
        }

        if(x2 % 2 == 0){
            x2o = x2 - 1;
            x2e = x2;
        } else{
            x2o = x2;
            x2e = x2 - 1;
        }

        if(y2 % 2 == 0){
            y2o = y2 - 1;
            y2e = y2;
        } else{
            y2o = y2;
            y2e = y2 - 1;
        }

        long total = 0;
        total += (1 + ((y2o - y1o) / 2)) * (1 + (x2o - x1o) / 2);
        total += (1 + ((y2e - y1e) / 2)) * (1 + (x2e - x1e) / 2);

        pw.println(total);

        pw.close();
    }
}

/**
*                _        _                 _                                                
*               | |      | |               | |                                               
*   ___ ___   __| | ___  | |__  _   _    __| | __ _ _ __ _ __ ___ _ __     _   _  __ _  ___  
*  / __/ _ \ / _` |/ _ \ | '_ \| | | |  / _` |/ _` | '__| '__/ _ \ '_ \   | | | |/ _` |/ _ \ 
* | (_| (_) | (_| |  __/ | |_) | |_| | | (_| | (_| | |  | | |  __/ | | |  | |_| | (_| | (_) |
*  \___\___/ \__,_|\___| |_.__/ \__, |  \__,_|\__,_|_|  |_|  \___|_| |_|   \__, |\__,_|\___/ 
*                                __/ |                               ______ __/ |            
*                               |___/                               |______|___/             
 */