#include <bits/stdc++.h>
using namespace std;
int main (){
    int a,b,c,d,min,max;
    cin >> a >> b >> c >> d;
    if(b<c || d<a){
        cout << (b-a+1)*(d-c+1) << endl;
    }
    else if(c<=b && b<=d){
        cout << (b-a+1)*(d-c+1)-(b-c+1) << endl;
    }
    else{
        cout <<  (b-a+1)*(d-c+1)-(d-a+1) << endl;
    }
}