-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.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Calendar;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.SortedSet;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.TreeSet;

/**
 * #
 * @author pttrung
 */
public class E_Volbit {

    public static long MOD = 1000000007;

    public static void main(String[] args) throws FileNotFoundException {
        // PrintWriter out = new PrintWriter(new FileOutputStream(new File(
        // "output.txt")));
        PrintWriter out = new PrintWriter(System.out);
        Scanner in = new Scanner();
        long x1 = in.nextInt();
        long y1 = in.nextInt();
        long x2 = in.nextInt();
        long y2 = in.nextInt();
        long num = (x2 - x1) + 1;
        long odd = (x1 % 2 == 0 ? 0 : 1) + num / 2;
        long even = num - odd;
        long a = 0, b= 0;
        if(y1 % 2 == 0){
            a = (y2 - y1)/2;
            b = (y2 - y1 )/2 + 1;
        }else{
            b = (y2 - y1)/2;
            a = (y2 - y1 )/2 + 1;
        }
      //  System.out.println(odd + " " + even + " " + a + " " + b);
        long result = odd*a + even*b;
        out.println(result);
        out.close();
    }

    public static int[] KMP(String val) {
        int i = 0;
        int j = -1;
        int[] result = new int[val.length() + 1];
        result[0] = -1;
        while (i < val.length()) {
            while (j >= 0 && val.charAt(j) != val.charAt(i)) {
                j = result[j];
            }
            j++;
            i++;
            result[i] = j;
        }
        return result;

    }

    public static boolean nextPer(int[] data) {
        int i = data.length - 1;
        while (i > 0 && data[i] < data[i - 1]) {
            i--;
        }
        if (i == 0) {
            return false;
        }
        int j = data.length - 1;
        while (data[j] < data[i - 1]) {
            j--;
        }
        int temp = data[i - 1];
        data[i - 1] = data[j];
        data[j] = temp;
        Arrays.sort(data, i, data.length);
        return true;
    }

    public static int digit(long n) {
        int result = 0;
        while (n > 0) {
            n /= 10;
            result++;
        }
        return result;
    }

    public static double dist(long a, long b, long x, long y) {
        double val = (b - a) * (b - a) + (x - y) * (x - y);
        val = Math.sqrt(val);
        double other = x * x + a * a;
        other = Math.sqrt(other);
        return val + other;

    }

    public static class Point implements Comparable<Point> {

        int x, y;

        public Point(int start, int end) {
            this.x = start;
            this.y = end;
        }

        @Override
        public int hashCode() {
            int hash = 5;
            hash = 47 * hash + this.x;
            hash = 47 * hash + this.y;
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final Point other = (Point) obj;
            if (this.x != other.x) {
                return false;
            }
            if (this.y != other.y) {
                return false;
            }
            return true;
        }

        @Override
        public int compareTo(Point o) {
            return x - o.x;
        }
    }

    public static class FT {

        long[] data;

        FT(int n) {
            data = new long[n];
        }

        public void update(int index, long value) {
            while (index < data.length) {
                data[index] += value;
                index += (index & (-index));
            }
        }

        public long get(int index) {
            long result = 0;
            while (index > 0) {
                result += data[index];
                index -= (index & (-index));
            }
            return result;

        }
    }

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

    public static long pow(long a, long b, long MOD) {
        if (b == 0) {
            return 1;
        }
        if (b == 1) {
            return a;
        }
        long val = pow(a, b / 2, MOD);
        if (b % 2 == 0) {
            return val * val % MOD;
        } else {
            return val * (val * a % MOD) % MOD;

        }
    }

    static class Scanner {

        BufferedReader br;
        StringTokenizer st;

        public Scanner() throws FileNotFoundException {
            // System.setOut(new PrintStream(new BufferedOutputStream(System.out), true));
            br = new BufferedReader(new InputStreamReader(System.in));
            //  br = new BufferedReader(new InputStreamReader(new FileInputStream(new File("input.txt"))));
        }

        public String next() {

            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    throw new RuntimeException();
                }
            }
            return st.nextToken();
        }

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

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

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

        public String nextLine() {
            st = null;
            try {
                return br.readLine();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }

        public boolean endLine() {
            try {
                String next = br.readLine();
                while (next != null && next.trim().isEmpty()) {
                    next = br.readLine();
                }
                if (next == null) {
                    return true;
                }
                st = new StringTokenizer(next);
                return st.hasMoreTokens();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }
    }
}
