/* -*- coding: utf-8 -*-
 *
 * 799.cc:  No.799 赤黒かーどげぇむ - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);

  int cnt = (b - a + 1) * (d - c + 1);
  if (a <= d && c <= b)
    cnt -= min(b, d) - max(a, c) + 1;

  printf("%d\n", cnt);
  return 0;
}