#include <bits/stdc++.h>
using namespace std;
long long odd, even;
long long nodd, neven;
int main() {
  long long x1, y1, x2, y2;
  scanf("%I64d%I64d%I64d%I64d", &x1, &y1, &x2, &y2);
  y2 -= y1;
  x2 -= x1;
  x2++;
  if (y2 % 2 == 0) {
    odd = y2 / 2 + 1;
    even = y2 / 2;
  } else {
    odd = (y2 + 1) / 2;
    even = y2 / 2 + 1;
  }
  if (x2 % 2 == 0) {
    nodd = x2 / 2;
    neven = x2 - nodd;
  } else {
    nodd = (x2 + 1) / 2;
    neven = x2 - nodd;
  }
  printf("%I64d\n", odd * nodd + even * neven);
  return 0;
}
