#include <bits/stdc++.h>
using namespace std;
long long X1, Y1, X2, Y2, xx, yy;
int main() {
  cin >> X1 >> Y1 >> X2 >> Y2;
  xx = (Y2 - Y1) / 2 + 1;
  yy = (Y2 - Y1 + 1) / 2;
  cout << xx * ((X2 - X1) / 2 + 1) + yy * (X2 - X1) / 2 << endl;
  return 0;
}
