#define _USE_MATH_DEFINES

#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max,count
#include <string>	//string,getline, to_string
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap, pair
#include <cstdlib>	//abs(int)
#include <cmath>	//sqrt,ceil,M_PI, pow, sin
#include <sstream>	//stringstream,getline
#include <numeric>	//gcd, accumlate
#include <deque>	//deque
#include <random>	//randam_device
#include <limits>	//numeric_limits

using namespace std;
constexpr long long int D_MOD = 1000000007;

int main() {

	int a, b, c, d;
	cin >> a >> b >> c >> d;

	int ans = 0;

	for (int i = a; i <= b; i++) {
		for (int j = c; j <= d; j++) {
			if (i != j) {
				ans++;
			}
		}
	}

	cout << ans << endl;

	return 0;

}