#include <iostream>
using namespace std;

int main() {
    int A,B,C,D;
    cin>>A>>B>>C>>D;
    int interSection = 0;
    for (int i = A; i <= B; i++) {
        if (C <= i && i <= D) interSection++;
    }
    cout << ((B - A + 1) * (D - C + 1)) - interSection << endl;
    return 0;
}