//#define NDEBUG
#include <cstddef>
#include <cstdint>
#include <vector>

namespace n91 {

	using i8 = std::int_least8_t;
	using i32 = std::int_least32_t;
	using i64 = std::int_least64_t;
	using u8 = std::uint_least8_t;
	using u32 = std::uint_least32_t;
	using u64 = std::uint_least64_t;
	using isize = std::ptrdiff_t;
	using usize = std::size_t;

	class rep {
		const usize f, l;

	public:
		class itr {
			friend rep;
			usize i;
			constexpr itr(const usize x) noexcept : i(x) {}

		public:
			void operator++() noexcept { ++i; }
			constexpr usize operator*() const noexcept { return i; }
			constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
		};
		constexpr rep(const usize first, const usize last) noexcept
			: f(first), l(last) {}
		constexpr itr begin() const noexcept { return itr(f); }
		constexpr itr end() const noexcept { return itr(l); }
	};
	class revrep {
		const usize f, l;

	public:
		class itr {
			friend revrep;
			usize i;
			constexpr itr(usize x) noexcept : i(x) {}

		public:
			void operator++() noexcept { --i; }
			constexpr usize operator*() const noexcept { return i; }
			constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
		};
		constexpr revrep(usize first, usize last) noexcept : f(--first), l(--last) {}
		constexpr itr begin() const noexcept { return itr(l); }
		constexpr itr end() const noexcept { return itr(f); }
	};
	template <class T> using vec_alias = std::vector<T>;
	template <class T> auto md_vec(const usize n, const T &value) {
		return std::vector<T>(n, value);
	}
	template <class... Args> auto md_vec(const usize n, Args... args) {
		return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
	}
	template <class T> constexpr T difference(const T &a, const T &b) {
		return a < b ? b - a : a - b;
	}

} // namespace n91

#include <cassert>
#include <cstdint>

namespace n91 {

	template <std::uint_least32_t Modulus> class modint {
		using u32 = std::uint_least32_t;
		using u64 = std::uint_least64_t;
		using i64 = std::int_fast64_t;
		class optimize_tag_t {};
		static constexpr optimize_tag_t optimize_tag{};

	public:
		using value_type = u32;
		value_type a;

	private:
		static constexpr value_type cst(i64 x) noexcept {
			x %= static_cast<i64>(Modulus);
			if (x < static_cast<i64>(0)) {
				x += static_cast<i64>(Modulus);
			}
			return static_cast<value_type>(x);
		}

		constexpr modint(optimize_tag_t, const value_type &x) noexcept : a(x) {}

	public:
		constexpr modint() noexcept : a(static_cast<value_type>(0)) {}
		constexpr modint(const i64 &x) noexcept : a(cst(x)) {}
		constexpr value_type value() const noexcept { return a; }
		constexpr modint operator+(const modint &o) const noexcept {
			return modint(optimize_tag,
				a + o.a < Modulus ? a + o.a : a + o.a - Modulus);
		}
		constexpr modint operator-(const modint &o) const noexcept {
			return modint(optimize_tag, a < o.a ? a + Modulus - o.a : a - o.a);
		}
		constexpr modint operator*(const modint &o) const noexcept {
			return modint(
				optimize_tag,
				static_cast<value_type>(static_cast<u64>(a) * static_cast<u64>(o.a) %
					static_cast<u64>(Modulus)));
		}
		constexpr modint operator/(const modint &o) const {
			return modint(
				optimize_tag,
				static_cast<value_type>(static_cast<u64>(a) * static_cast<u64>((~o).a) %
					static_cast<u64>(Modulus)));
		}
		modint &operator+=(const modint &o) noexcept {
			if ((a += o.a) >= Modulus)
				a -= Modulus;
			return *this;
		}
		modint &operator-=(const modint &o) noexcept {
			if (a < o.a)
				a += Modulus;
			a -= o.a;
			return *this;
		}
		modint &operator*=(const modint &o) noexcept {
			a = static_cast<value_type>(static_cast<u64>(a) * static_cast<u64>(o.a) %
				static_cast<u64>(Modulus));
			return *this;
		}
		modint &operator/=(const modint &o) {
			a = static_cast<u64>(a) * (~o).a % Modulus;
			return *this;
		}
		constexpr modint inverse() const noexcept {
			assert(a != static_cast<value_type>(0) && "0 does not have inverse");
			return pow(static_cast<u64>(Modulus - static_cast<value_type>(2)));
		}
		constexpr modint operator~() const noexcept { return inverse(); }
		constexpr modint operator-() const noexcept {
			if (a == static_cast<value_type>(0)) {
				return modint(optimize_tag, static_cast<value_type>(0));
			}
			else {
				return modint(optimize_tag, Modulus - a);
			}
		}
		modint &operator++() noexcept {
			if (++a == Modulus) {
				a = static_cast<value_type>(0);
			}
			return *this;
		}
		modint &operator--() noexcept {
			if (a == static_cast<value_type>(0)) {
				a = Modulus;
			}
			--a;
			return *this;
		}
		constexpr bool operator==(const modint &o) const noexcept { return a == o.a; }
		constexpr bool operator!=(const modint &o) const noexcept { return a != o.a; }
		constexpr bool operator<(const modint &o) const noexcept { return a < o.a; }
		constexpr bool operator<=(const modint &o) const noexcept { return a <= o.a; }
		constexpr bool operator>(const modint &o) const noexcept { return a > o.a; }
		constexpr bool operator>=(const modint &o) const noexcept { return a >= o.a; }
		constexpr explicit operator bool() const noexcept { return a; }
		constexpr explicit operator value_type() const noexcept { return a; }
		modint pow(u64 x) const noexcept {
			u64 t = a, u = 1;
			while (x) {
				if (x & 1)
					u = u * t % Modulus;
				t = (t * t) % Modulus;
				x >>= 1;
			}
			return modint(optimize_tag, static_cast<value_type>(u));
		}
	};

} // namespace n91


#include <algorithm>
#include <iostream>
#include <utility>

namespace n91 {

	void main_() {
		u32 a, b, c, d;
		std::cin >> a >> b >> c >> d;
		--a;
		--c;
		u32 ans = (b - a)*(d - c);
		if (c <= b&&a <= d) {
			ans -= std::min(b, d) - std::max(a, c);
		}
		std::cout << ans << std::endl;
	}

} // namespace n91

int main() {
	n91::main_();
	return 0;
}