#include <bits/stdc++.h>
using namespace std;

typedef long long int ll;
ll MOD = 1000000007;
ll INFL = 1ll << 60;
ll INF = 1 << 30;

// ===============================================================================

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int ans = 0;
  for (int i = a; i <= b; i++) {
    for (int j = c; j <= d; j++) {
      if (i != j) ans++;
    }
  }
  cout << ans << endl;
}