#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

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