#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<climits>
#include<string>
#include<algorithm>
#include<vector>
#include<queue>
#include<stack>
#include<numeric>
#include<map>
#include<set>
#include<bitset>
using namespace std;
#define REP(i,m,n) for(int i = m; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a)  (a).rbegin(),(a).rend()
#define dup(x,y) (((x)+(y)-1) / (y)); //整数型のまま切り上げ演算する
#define PI 3.14159265359
#define MOD 1000000007
typedef long long LL;
template<class T> inline void chmax(T& a, T b) { if (a < b) { a = b; } }
template<class T> inline void chmin(T& a, T b) { if (a > b) { a = b; } }
template<class T> void print_vector(vector<T> vec) { for (int i = 0; i < vec.size(); i++) { cout << vec[i] << " "; } }


int main(void) {
	LL A, B, C, D;
	cin >> A >> B >> C >> D;

	LL count = 0;
	for (LL i = A; i <= B; i++)
	{
		for (LL j = C; j <= D; j++)
		{
			if (i!=j) {
				count++;
			}
		}
	}
	cout << count << endl;
	return 0;
}