#define _CRT_SECURE_NO_WARNINGS
#define _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include <map>
#include <fstream>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <assert.h>
#include <complex>
#include <unordered_map>
using namespace std;
#define eps 0.000000001
#define LONG_INF 10000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 998244353
#define seg_size 65536*2
#define REP(i,n) for(long long i = 0;i < n;++i)
unsigned long xor128(void)
{
	static unsigned long x = time(NULL), y = 362436069, z = 521288629, w = 88675123;
	unsigned long t;
	t = (x ^ (x << 11)); x = y; y = z; z = w; return(w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	cout << (b - a + 1) * (d - c + 1) - max(0, c - b + 1) << endl;
	return 0;
}