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

int main(){
    int A, B, C, D;
    cin >> A >> B >> C >> D;
    int ans = (B-A+1) * (D-C+1);
    int same = min(B, D) - max(A, C) + 1;
    if(same > 0) ans -= same;
    cout << ans << endl;
    return 0;
}