#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e16;
const long long MOD = 998244353;
const long double PI = 3.14159265359;
template <class T>
ostream& operator<<(ostream& os, const vector<T>& v) {
  for (T i : v) os << i << ' ';
  return os;
}
template <class T>
ostream& operator<<(ostream& os, const set<T>& v) {
  for (T i : v) os << i << ' ';
  return os;
}
template <class T, class S>
ostream& operator<<(ostream& os, const pair<T, S>& v) {
  os << v.first << ' ' << v.second;
  return os;
}
template <class T, class S>
ostream& operator<<(ostream& os, const unordered_map<T, S>& v) {
  for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' ';
  return os;
}
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target( \
    "avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
clock_t time_p = clock();
void starboy1299() {
  time_p = clock() - time_p;
  cerr << "Time Taken : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}
const long long N = 2e5 + 5;
void solve() {
  long long a, b, c, d;
  cin >> a >> b >> c >> d;
  long long ans = 0;
  for (long long i = a; i <= b; i++) {
    long long low = i + b, high = i + c;
    long long l = c + 1, h = d + 1;
    if (low > h) {
      ans += (h - l + 1) * (high - low + 1);
      continue;
    }
    if (high < l) {
      continue;
    }
    long long nl = max(low, l);
    long long nr = min(high, h);
    long long k = nl - l + 1;
    ans += k * (nr - nl + 1) + ((nr - nl) * (nr - nl + 1)) / 2;
    if (high > h) {
      ans += (high - h) * (h - l + 1);
    }
  }
  cout << ans << '\n';
}
int32_t main() {
  fast();
  long long t = 1;
  for (long long i = 1; i <= t; i++) {
    solve();
  }
  starboy1299();
}
