#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int u = min(b, d);
  int l = max(a, c);
  if (u - l + 1 < 0) {
    cout << (b - a + 1) * (d - c + 1) << endl;
  } else {
    cout << (b - a + 1) * (d - c + 1) - (u - l + 1) << endl;
  }
  return 0;
}