#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;



int main() {

    int A, B, C, D;
    cin >> A >> B >> C >> D;
    int cnt = 0;
    for (int i = A; i <= B; ++i) {
        for (int j = C; j <= D; ++j) {
            if (i != j) {
                cnt += 1;
            }
        }
    }

    cout << cnt << endl;
    return 0;
}