-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.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;

public class Main {
    public static void main(String[] args) {
//     Test.testing();
        ConsoleIO io = new ConsoleIO();
        new Main(io).solve();
        io.flush();
    }
    ConsoleIO io;
    Main(ConsoleIO io) {
        this.io = io;
    }

    ArrayList<ArrayList<Integer>> gr1;
    boolean[] visit;
    //

    class Pair{
        public Pair(int a, int b){
            this.a = a;
            this.b = b;
        }
        public int a;
        public int b;
    }

    class Edge {
        public Edge(int c, int u,int v) {
            this.u = u;
            this.v = v;
            this.c = c;
        }
        public int u;
        public int v;
        public int c;
    }

    long MOD = 1_000_000_007;
    int n,m,k,fn;
    int[][] map;
    public void solve() {
        int[] l = io.readIntArray();
        long x= (l[2]-l[0])/2,y=(l[3]-l[1])/2;
        long res = x*y + (x+1)*(y+1) ;
        io.writeLine(res + "");
    }

    double pow(double a, long p) {
        double res = 1;
        while (p > 0) {
            if (p % 2 == 1) res = (res * a);
            a = (a * a);
            p /= 2;
        }
        return res;
    }

    long gcd(long a, long b) {
        if (a < b) return gcd(b, a);
        if (b == 0) return a;
        return gcd(b, a % b);
    }
}

class ConsoleIO {
    BufferedReader br;
    PrintWriter out;
    public ConsoleIO(){br = new BufferedReader(new InputStreamReader(System.in));out = new PrintWriter(System.out);}
    public void flush(){this.out.close();}
    public void writeLine(String s) {this.out.println(s);}
    public void writeInt(int a) {this.out.print(a);this.out.print(' ');}
    public void writeWord(String s){
        this.out.print(s);
    }
    public String readLine() {try {return br.readLine();}catch (Exception ex){ return "";}}
    public long readLong() {
        return Long.parseLong(this.readLine());
    }
    public int readInt() {
        return Integer.parseInt(this.readLine().trim());
    }
    public long[] readLongArray() {
        String[]n=this.readLine().trim().split(" ");long[]r=new long[n.length];
        for(int i=0;i<n.length;i++)r[i]=Long.parseLong(n[i]);
        return r;
    }
    public int[] readIntArray() {
        String[]n=this.readLine().trim().split("\\s+");int[]r=new int[n.length];
        for(int i=0;i<n.length;i++)r[i]=Integer.parseInt(n[i]);
        return r;
    }
    public void writeIntArray(int[] a) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < a.length; i++) {if (i > 0) sb.append(' ');sb.append(a[i]);}
        this.writeLine(sb.toString());
    }
}


