#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;

    int x = 0;
    int p[101] = {};
    for (int i = a; i <= b; i++) {
        p[i] = 1;
    }
    for (int i = c; i <= d; i++) {
        if (p[i]) x++;
    }

    cout << (b - a + 1) * (d - c + 1) - x << endl;

    return 0;
}