-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.*;
import java.math.BigInteger;
import java.util.Map.Entry;

import static java.lang.Math.*;

public class Solution extends PrintWriter {

	void run() {
		long x1 = nextInt(), y1 = nextInt();
		long x2 = nextInt(), y2 = nextInt();

		if (y2 < 0) {
			y2 = -y2;
			y1 = -y1;
		}
		if (y1 < 0) {
			long dy = ((-y1) / 2) * 2 + 8;
			y1 += dy;
			y2 += dy;
		}

		if (x2 < 0) {
			x2 = -x2;
			x1 = -x1;
		}
		if (x1 < 0) {
			long dx = ((-x1) / 2) * 2 + 8;
			x1 += dx;
			x2 += dx;
		}

		println(solve(x2, y2) - solve(x1 - 1, y2) - solve(x2, y1 - 1) + solve(x1 - 1, y1 - 1));
	}

	long solve(long x, long y) {
		long a = (x / 2) + 1;
		long b = x - a + 1;

		return a * ((y / 2) + 1) + b * ((y + 1) / 2);
	}

	int[][] nextMatrix(int n, int m) {
		int[][] matrix = new int[n][m];
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++)
				matrix[i][j] = nextInt();
		return matrix;
	}

	String next() {
		while (!tokenizer.hasMoreTokens())
			tokenizer = new StringTokenizer(nextLine());
		return tokenizer.nextToken();
	}

	boolean hasNext() {
		while (!tokenizer.hasMoreTokens()) {
			String line = nextLine();
			if (line == null) {
				return false;
			}
			tokenizer = new StringTokenizer(line);
		}
		return true;
	}

	int[] nextArray(int n) {
		int[] array = new int[n];
		for (int i = 0; i < n; i++) {
			array[i] = nextInt();
		}
		return array;
	}

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

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

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

	String nextLine() {
		try {
			return reader.readLine();
		} catch (IOException err) {
			return null;
		}
	}

	public Solution(OutputStream outputStream) {
		super(outputStream);
	}

	static BufferedReader reader;
	static StringTokenizer tokenizer = new StringTokenizer("");
	static Random rnd = new Random();

	public static void main(String[] args) throws IOException {
		Solution solution = new Solution(System.out);
		reader = new BufferedReader(new InputStreamReader(System.in));
		solution.run();
		solution.close();
		reader.close();
	}
}