#include <bits/stdc++.h>
using namespace std;
long long n, m, c, l, r, i, j, x1, x2, x3, x, r_y, r_y1, r_y2, r_y3, k, g, sum,
    maxy, maxx, qy, qx;
long long kk;
long long fact(long long a) {
  unsigned long long k = 1, i;
  for (i = 1; i <= a; i++) k *= i;
  return k;
}
int main() {
  cin >> x1 >> r_y1 >> x2 >> r_y2;
  x = x2 - x1;
  r_y = r_y2 - r_y1;
  maxy = (2 + r_y) / 2;
  qy = (2 + r_y) % 2;
  maxx = x + 1;
  qx = maxx % 2;
  if (qy == 1)
    cout << maxx * maxy;
  else {
    cout << (maxx / 2 + qx) * maxy + (maxx / 2) * (maxy - 1);
  }
}
