#include <bits/stdc++.h>
#ifdef _DEBUG
#include "debug.hpp"
#else
#define debug(...)
#endif
#define fastIO (cin.tie(0), cout.tie(0), ios::sync_with_stdio(false))
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define precise(i) fixed << setprecision(i)
using ill = int64_t;
using ull = uint64_t;
using namespace std;

void solve() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int cnt = 0;
  for (int i = a; i <= b; i++) {
    for (int j = c; j <= d; j++) {
      if (i != j)
        cnt++;
    }
  }
  cout << cnt << '\n';
}

void wrapper() {
  rep(i, 2) {
    cout << "[TEST " << i << "]\n";
    solve();
    cout << "\n";
  }
}

int main() {
  fastIO;
#ifdef _DEBUG
  wrapper();
#else
  solve();
#endif
}