//#include <atcoder/all>
#include <iostream>
#include <numeric>
#include <cmath>
#include <limits>
#include <stdio.h>
#include <iomanip>
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<pair<ll,ll>>>;

#define mp(A,B) make_pair(A,B)
bool OK(ll N) {
	ll k = 0;
	for (ll i = 2; i * i <= N; i++) {
		while(N % i == 0) {
			k++;
			N /= i;
		}
	}
	if (N == 1)k--;
	return (k > 1);
}

set<string> AN;
void dfs(string S, string nowS) {
	ll N = S.size();
	if (N == 1) {
		AN.insert(nowS + S);
		return;
	}
	else{
		dfs(S.substr(0, N - 1), nowS + S[N - 1]);
		dfs(S.substr(1, N - 1), nowS + S[0]);
	}
}

int main() {
	ll A, B, C, D, an = 0;
	cin >> A >> B >> C >> D;
	for (ll i = A; i <= B; i++) {
		for (ll j = C; j <= D; j++) {
			if (i != j)an++;
		}
	}
	cout << an << endl;
}