#include <cassert>
#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <set>
#include <queue>
#include <map>
#include <vector>
#include <algorithm>
#include <ctime>
#include <cstring>
#include <cmath>
#include <string>
#include <bitset>

#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,avx,avx2")

#include <cstring>
#include <unordered_map>
#include <map>
#include <algorithm>
#include <cstdio>
#include <set>
#include <vector>
#include <unordered_set>
#include <cassert>
#include <string>
#include <cmath>
#include <set>
#include <bitset>

using namespace std;

namespace olymplib {
	namespace {
		template <class T> void writeInt(T x);
		template <class T> void writeUInt(T x);
		void writeChar(int x);
		void writeWord(const char *s);
		void writeDouble(double x, int len = 0);
		static const int buf_size = 4096;
		static int write_pos = 0;
		static char write_buf[buf_size];
		static char buf[buf_size];
		static int buf_len = 0, buf_pos = 0;
		//-- read
		template <class T = int> T readInt();
		double readDouble();
		int readUInt();
		int readChar();
		void readWord(char *s);
		bool readLine(char *s); // do not save '\n'
		bool isEof();
		int peekChar();
		bool seekEof();
		void flush();
		//--------------------------
		bool isEof() {
			if (buf_pos == buf_len) {
				buf_pos = 0, buf_len = fread(buf, 1, buf_size, stdin);
				if (buf_pos == buf_len)
					return 1;
			}
			return 0;
		}

		int getChar() {
			return isEof() ? -1 : buf[buf_pos++];
		}

		int peekChar() {
			return isEof() ? -1 : buf[buf_pos];
		}

		bool seekEof() {
			int c;
			while ((c = peekChar()) != -1 && c <= 32)
				buf_pos++;
			return c == -1;
		}

		int readChar() {
			int c = getChar();
			while (c != -1 && c <= 32)
				c = getChar();
			return c;
		}

		int readUInt() {
			int c = readChar(), x = 0;
			while ('0' <= c && c <= '9')
				x = x * 10 + c - '0', c = getChar();
			return x;
		}

		template <class T>
		T readInt() {
			int s = 1, c = readChar();
			T x = 0;
			if (c == '-')
				s = -1, c = getChar();
			while ('0' <= c && c <= '9')
				x = x * 10 + c - '0', c = getChar();
			return s == 1 ? x : -x;
		}

		template <class Double = double>
		double readDouble() {
			int s = 1, c = readChar();
			Double x = 0;
			if (c == '-')
				s = -1, c = getChar();
			while ('0' <= c && c <= '9')
				x = x * 10 + c - '0', c = getChar();
			if (c == '.') {
				c = getChar();
				double coef = 1;
				while ('0' <= c && c <= '9')
					x += (c - '0') * (coef *= 1e-1), c = getChar();
			}
			return s == 1 ? x : -x;
		}

		void readWord(char *s) {
			int c = readChar();
			while (c > 32)
				*s++ = c, c = getChar();
			*s = 0;
		}

		bool readLine(char *s) {
			int c = getChar();
			while (c != '\n' && c != -1)
				*s++ = c, c = getChar();
			*s = 0;
			return c != -1;
		}

		//only supports fixed precision for now (hence huge values are not allowed)
		void writeDouble(double x, int output_len) {
			if (x < 0)
				writeChar('-'), x = -x;
			int t = (int)x;
			writeUInt(t), x -= t;
			writeChar('.');
			for (int i = output_len - 1; i > 0; i--) {
				x *= 10;
				t = 9 < (int)x ? 9 : x;
				writeChar('0' + t), x -= t;
			}
			x *= 10;
			t = 9 < (int)(x + 0.5) ? 9 : (int)(x + 0.5);
			writeChar('0' + t);
		}

		void writeChar(int x) {
			if (write_pos == buf_size)
				fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
			write_buf[write_pos++] = x;
		}

		template <class T>
		void writeUInt(T x) {
			char s[24];
			int n = 0;
			while (x || !n)
				s[n++] = '0' + x % 10, x /= 10;
			while (n--)
				writeChar(s[n]);
		}

		template<>
		void writeInt<int>(int x) {
			if (x < 0)
				writeChar('-'), x = -x;

			char s[10];
			int n = 0;
			while (x || !n)
				s[n++] = '0' + x % 10, x /= 10;
			while (n--)
				writeChar(s[n]);
		}

		template <class T>
		void writeInt(T x) {
			if (x < 0)
				writeChar('-'), x = -x;

			char s[24];
			int n = 0;
			while (x || !n)
				s[n++] = '0' + x % 10, x /= 10;
			while (n--)
				writeChar(s[n]);
		}

		void writeWord(const char *s) {
			while (*s)
				writeChar(*s++);
		}

		void flush() {
			if (write_pos) {
				fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
			}
		}
	}

	class TOutputStream {
		int precision_;
	public:
		void precision(int new_prec) {
			precision_ = new_prec;
		}

		TOutputStream& operator<< (bool arg) {
			writeChar('0' + arg);
			return *this;
		}

		TOutputStream& operator<< (char arg) {
			writeChar(arg);
			return *this;
		}

		TOutputStream& operator<< (const char* arg) {
			writeWord(arg);
			return *this;
		}

		TOutputStream& operator << (float arg) {
			writeDouble(arg, precision_);
			return *this;
		}

		TOutputStream& operator << (double arg) {
			writeDouble(arg, precision_);
			return *this;
		}

		TOutputStream& operator << (long double arg) {
			writeDouble(arg, precision_);
			return *this;
		}

		TOutputStream& operator<< (int arg) {
			writeInt(arg);
			return *this;
		}

		TOutputStream& operator<< (long long arg) {
			writeInt(arg);
			return *this;
		}

		TOutputStream& operator<< (const std::string& arg) {
			writeWord(arg.c_str());
			return *this;
		}

		void flush() {
			olymplib::flush();
		}

		typedef TOutputStream& (*MyStreamManipulator)(TOutputStream&);

		TOutputStream& operator<<(const MyStreamManipulator& manip) {
			return manip(*this);
		}

		~TOutputStream() {
			flush();
		}
	};

	class TInputStream {
	public:
		TInputStream& operator>> (bool& arg) {
			arg = readChar() - '0';
			return *this;
		}

		TInputStream& operator>> (char& arg) {
			arg = readChar();
			return *this;
		}

		//please know what you're doing.
		TInputStream& operator>> (char*& arg) {
			readWord(arg);
			return *this;
		}

		TInputStream& operator >> (float& arg) {
			arg = readDouble<float>();
			return *this;
		}

		TInputStream& operator >> (double& arg) {
			arg = readDouble<double>();
			return *this;
		}

		TInputStream& operator >> (long double& arg) {
			arg = readDouble<long double>();
			return *this;
		}

		TInputStream& operator>> (int& arg) {
			arg = readInt<int>();
			return *this;
		}

		TInputStream& operator>> (long long& arg) {
			arg = readInt<long long>();
			return *this;
		}

		TInputStream& operator>> (std::string& arg) {
			arg.clear();
			arg.reserve(16);
			arg.push_back(readChar());
			while (char c = peekChar()) {
				if (c == -1 || c <= 32)
					break;
				arg.push_back(getChar());
			}
			arg.shrink_to_fit();
			return *this;
		}
	};

	static TInputStream cin;
	static TOutputStream cout;

	static TOutputStream& endl(TOutputStream& stream) {
		stream << '\n';
		stream.flush();
		return stream;
	}
}

using namespace std;

const int N = 1e5;

int main() {
	int H, W, h, w;
	cin >> H >> W >> h >> w;
	cout << (H - h) * (W - w) << endl;
	return 0;
}

Measured memory_bytes: 256