#include <bits/stdc++.h>
using namespace std;
long long ceill(long long a, long long b) { return a / b + bool(a % b); }
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
long long power(long long x, long long y) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = (res * x) % 1000000007;
    x = (x * x) % 1000000007;
    y >>= 1;
  }
  return res;
}
void bin(unsigned n, string &str) {
  str = "";
  if (n > 1) bin(n >> 1, str);
  str += to_string(n & 1);
}
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (long long i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
void primeFactors(long long n, vector<long long> &vec) {
  while (n % 2 == 0) vec.push_back(2), n /= 2;
  for (long long i = 3; i <= sqrt(n); i += 2)
    while (n % i == 0) vec.push_back(i), n /= i;
  if (n > 2) vec.push_back(n);
}
void solve() {
  long long a, b, c, d;
  cin >> a >> b >> c >> d;
  long long k = a + b, ans = 0;
  if (k <= c) k = c + 1;
  for (long long i = k; i <= b + c; i++) {
    long long lz = c, rz = min(i - 1, d);
    long long ly = max(i - b, b), ry = min(i - a, c);
    ans += (rz - lz + 1) * (ry - ly + 1);
  }
  cout << ans << '\n';
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long tc = 1;
  for (long long TTT = 1; TTT <= tc; TTT++) {
    solve();
  }
}
