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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int A, B, C, D;
    cin >> A >> B >> C >> D;
    int head, tail;
    head = min(B, D);
    tail = max(A, C);
    int overlap = 0;
    if (head >= tail) overlap = head - tail + 1;
    int ans = (B - A + 1) * (D - C + 1) - overlap;
    cout << ans << endl;
    return 0;
}