#include <iostream> using namespace std; int a, b, c, d; int main(void) { cin >> a >> b >> c >> d; if(b < c || d < a){ cout<<(b-a+1)*(d-c+1) << endl; } else{ cout << (b-a+1)*(d-c+1) - (min(b, d)-max(a, c)+1) << endl; } return 0; }