#include<bits/stdc++.h>
using namespace std;

int main(){
    long A, B, C, D;
    cin >> A >> B >> C >> D;
    ++B;
    ++D;

    long ans = (B - A) * (D - C) - max(min(B, D) - max(A, C), 0L);

    cout << ans << endl;

    return 0;
}