#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); } }