#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); i++)

const int MOD = 1e9 + 7;

struct mint {
    int n;
    mint(int n_ = 0) : n(n_) {}
};

mint operator+(mint a, mint b) { a.n += b.n; if (a.n >= MOD) a.n -= MOD; return a; }
mint operator-(mint a, mint b) { a.n -= b.n; if (a.n < 0) a.n += MOD; return a; }
mint operator*(mint a, mint b) { return (long long)a.n * b.n % MOD; }
mint &operator+=(mint &a, mint b) { return a = a + b; }
mint &operator-=(mint &a, mint b) { return a = a - b; }
mint &operator*=(mint &a, mint b) { return a = a * b; }

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int ans = (b - a + 1) * (d - c + 1);
  int l = max(a, c);
  int r = min(b, d);
  if (l <= r) ans -= r - l + 1;
  cout << ans << '\n';
}