-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.util.*;
import java.io.*;
import java.math.BigInteger;
import java.text.*;
public class Main {
    static long mod = 1000_000_007;
    static long mod1 = 998244353;
    static boolean fileIO = true;
    static boolean memory = true;
    static FastScanner f;
    static PrintWriter pw;
    static double eps = (double)1e-6;
    static int oo = (int)1e7;

    public static void solve() throws Exception {
        long x1 = f.nl(); long y1 = f.nl();
        long x2 = f.nl(); long y2 = f.nl();
        long t1 = x2 - x1 + 1;
        long t2 = ((y2 - y1) / 2) + 1;
        long t3 = (x2 - x1) / 2;
        long ans = t2 * t1 - t3;
        pn(ans);
    }
 
    public static void main(String[] args)throws Exception {
        if(memory) new Thread(null, new Runnable() {public void run(){try{new Main().run();}catch(Exception e){e.printStackTrace();System.exit(1);}}}, "", 1 << 28).start();
        else new Main().run();
    }
        
/******************************END OF MAIN PROGRAM*******************************************/
    void run()throws Exception {
        if (System.getProperty("ONLINE_JUDGE") == null) {
            f = new FastScanner("");
            pw = new PrintWriter(System.out);
        }
        else {
            f = new FastScanner();
            pw = new PrintWriter(System.out);
            //fw = new FileWriter("!out.txt");
        }
        //pre();
        int t = 1;
        int tt = 1;
        while(t --> 0) {
            //fw.write("Case #" + (tt++) + ": ");
            //fw.write("\n");
            solve();
        }
        pw.flush(); 
        pw.close();
        //fw.close();  
    }

    public static class FastScanner {
        BufferedReader br;
        StringTokenizer st;
 
        public FastScanner(String str) throws Exception {
            try {
                br = new BufferedReader(new FileReader("!a.txt"));
            }
            catch (Exception e) {
                e.printStackTrace();
            }
        }
 
        public FastScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }
 
        public String next()throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }
 
        public int ni() throws IOException {return Integer.parseInt(next());}
 
        public long nl() throws IOException {return Long.parseLong(next());}
 
        public String nextLine() throws IOException {return br.readLine();}
 
        public double nd() throws IOException {return Double.parseDouble(next());}
 
    }
 
    public static void pn(Object... o) {for(int i = 0; i < o.length; ++i) pw.print(o[i] + (i + 1 < o.length ? " ": "\n"));}
    public static void p(Object... o) {for(int i = 0; i < o.length; ++i) pw.print(o[i] + (i + 1 < o.length ? " " : ""));}
    public static void pni(Object... o) {for(Object obj : o) pw.print(oo + " "); pw.println(); pw.flush();}
    public static int gcd(int a,int b){if(b==0)return a;else{return gcd(b,a%b);}}
    public static long gcd(long a,long b){if(b==0l)return a;else{return gcd(b,a%b);}}
    public static long lcm(long a,long b){return (a*b/gcd(a,b));}
    public static long pow(long a,long b){long res=1;while(b>0){if((b&1)==1)res=res*a;b>>=1;a=a*a;}return res;}
    public static int pow(int a,int b){int res=1;while(b>0){if((b&1)==1)res=res*a;b>>=1;a=a*a;}return res;}
    public static long mpow(long a,long b, long m){long res=1;while(b>0){if((b&1)==1)res=((res%m)*(a%m))%m;b>>=1;a=((a%m)*(a%m))%m;}return res;}
    public static long mul(long a , long b , long mod){return ((a%mod)*(b%mod)%mod);}
    public static long adp(long a , long b){return ((a%mod)+(b%mod)%mod);}
    public static int dig(long a){int cnt=0;while(a>0){a/=10;++cnt;}return Math.max(1,cnt);}
    public static int dig(int a){int cnt=0;while(a>0){a/=10;++cnt;}return Math.max(1,cnt);}
    public static boolean isPrime(long n){if(n<=1)return false;if(n<=3)return true;if(n%2==0||n%3==0)return false;for(long i=5;i*i<=n;i=i+6)if(n%i==0||n%(i+2)==0)return false;return true;}
    public static boolean isPrime(int n){if(n<=1)return false;if(n<=3)return true;if(n%2==0||n%3==0)return false;for(int i=5;i*i<=n;i=i+6)if(n%i==0||n%(i+2)==0)return false;return true;}
    public static HashSet<Long> factors(long n){HashSet<Long> hs=new HashSet<Long>();for(long i=1;i<=(long)Math.sqrt(n);i++){if(n%i==0){hs.add(i);hs.add(n/i);}}return hs;}
    public static HashSet<Integer> factors(int n){HashSet<Integer> hs=new HashSet<Integer>();for(int i=1;i<=(int)Math.sqrt(n);i++){if(n%i==0){hs.add(i);hs.add(n/i);}}return hs;}
    public static HashSet<Long> pf(long n){HashSet<Long> ff=factors(n);HashSet<Long> ans=new HashSet<Long>();for(Long i:ff)if(isPrime(i))ans.add(i);return ans;}
    public static HashSet<Integer> pf(int n){HashSet<Integer> ff=factors(n);HashSet<Integer> ans=new HashSet<Integer>();for(Integer i:ff)if(isPrime(i))ans.add(i);return ans;}
    public static int gnv(char c){return Character.getNumericValue(c);}
    public static void sort(int[] a){ArrayList<Integer> l=new ArrayList<>();for(int i:a)l.add(i);Collections.sort(l);for(int i=0;i<a.length;++i)a[i]=l.get(i);}
    public static void sort(long[] a){ArrayList<Long> l=new ArrayList<>();for(long i:a)l.add(i);Collections.sort(l);for(int i=0;i<a.length;++i)a[i]=l.get(i);}
    public static void sort(ArrayList<Integer> a){Collections.sort(a);}
}