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