#include<iostream>

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++) {
        if(i<c || i>d) {
            cnt += d-c+1;
        } else {
            cnt += d-c;
        }
    }

    cout << cnt << endl;
}