#include <iostream>
using namespace std;

int main() {
  int A, B, C, D; cin >> A >> B >> C >> D;
  if(B < C || D < A) {
    cout << (B-A+1) * (D-C+1) << endl;
  } else if(C <= B) {
    cout << (B-A+1) * (D-C+1) - (B-C+1) << endl;
  } else {
    cout << (B-A+1) * (D-C+1) - (D-A+1) << endl;
  }
  return 0;
}