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

int main () {
    int a,b,c,d;
    cin >> a >> b >> c >> d;
    if(c<=b){
        cout << (b-a+1)*(d-c+1)-(b-c+1);
    }else{
        cout << (b-a+1)*(d-c+1);
    }
}