#include <bits/stdc++.h>
using namespace std;
long long X1, Y1, X2, Y2;
long long ans = 0;
int main() {
  cin >> X1 >> Y1 >> X2 >> Y2;
  X1 += 2e9;
  Y1 += 2e9;
  X2 += 2e9;
  Y2 += 2e9;
  if ((Y2 - Y1) & 1) {
    ans = (X2 - X1 + 1) * (Y2 - Y1 + 1) / 2;
  } else {
    ans = (X2 - X1 + 1) * (Y2 - Y1 + 1) / 2;
    ans++;
  }
  cout << ans << endl;
  return 0;
}
