#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <limits>
#include <algorithm>
#include <queue>
#include <set>
#include <unordered_map>
#include <numeric>
#include <cmath>
#include <iomanip>
#include <stack>
#include <string>
#include <functional>
using namespace std;

using i64 = long long;
using ui64 = unsigned long long;

using P = pair<int, int>;

constexpr int INF = (1 << 30);
constexpr std::int64_t INF64 = (1LL << 60);
constexpr int MOD = 1e9 + 7;
//constexpr std::int64_t MOD64 = 1e11 + 3;

int dx[] = { -1, 0, 1, 0 };
int dy[] = { 0, 1, 0, -1 };

template <typename T, typename U>
static inline std::vector<U> makeNdVector(T n, U val) noexcept {
	static_assert(std::is_integral<T>::value, "[makeNdVector] 1st argument must be an integer type");
	return std::vector<U>(std::forward<T>(n), std::forward<U>(val));
}

template <typename T, typename ...Args>
static inline decltype(auto) makeNdVector(T n, Args... args) noexcept {
	static_assert(std::is_integral<T>::value, "[makeNdVector] 1st argument must be an integer type");
	return std::vector<decltype(makeNdVector(std::forward<Args>(args)...))>(std::forward<T>(n), makeNdVector(std::forward<Args>(args)...));
}

template <typename T>
void print(vector<T> vec)
{
	for (auto &e : vec)
		cout << e << " ";
	cout << endl;
}

void YES(bool f) { cout << (f ? "YES" : "NO") << endl; }
void Yes(bool f) { cout << (f ? "Yes" : "No") << endl; }
void yes(bool f) { cout << (f ? "yes" : "no") << endl; }

template <typename T>
T gcd(T x, T y) {
	if (x == 0 || y == 0) return 0;
	T r;
	while ((r = x % y) != 0) {
		x = y;
		y = r;
	}
	return y;
}

template <typename T>
T lcm(T x, T y) {
	if (x == 0 || y == 0) return 0;
	return (x / gcd(x, y) * y);
}

int next_combination(int sub)
{
	int x = sub & -sub, y = sub + x;
	return (((sub & ~y) / x) >> 1) | y;
}

std::uint64_t combination(int n, int r) {
	if (n < 0 || r < 0) return 0;
	if (n < r) return 0;
	std::uint64_t k = 1;
	for (std::uint64_t d = 1; d <= r; ++d) {
		k *= n--;
		k /= d;
	}
	return k;
}

bool isOK(i64 b, i64 n, i64 key) {
	i64 sum = 0;
	while (n > 0) {
		sum += n % b;
		n /= b;
	}
	if (sum == key) return true;
}

i64 binary_search(i64 n, i64 key) {
	i64 left = -1;
	i64 right = n + 1;
	while (right - left > 1) {
		i64 mid = left + (right - left) / 2;
		long double l = log(n) / log(mid);
		i64 num = floor(l) + 1;
		cout << mid << " " << num << endl;
		num *= (mid - 1);
		if (num >= key) right = mid;
		else left = mid;
	}
	return right;
}

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	using P = pair<int, int>;

	i64 A, B, C, D; cin >> A >> B >> C >> D;
	i64 sum = 0;
	for (int i = A; i <= B; ++i) {
		if (C <= i && i <= D) {
			sum += D - C;
		}
		else sum += D - C + 1;
	}

	cout << sum << endl;
}