#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);     }   }   