#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
using namespace std;
#define MOD (long long int)(1e9+7)
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)


int main(void){
	int A,B,C,D;
	cin>>A>>B>>C>>D;
	ll ans = 0;
	ll kyo = 0;
	for(int i=A; i<=B; i++){
		for(int j=C; j<=D; j++){
			if(i==j){
				kyo++;
			}
		}
	}
	ans += kyo*(kyo-1);
	ans += (B-A+1-kyo) * kyo;
	ans += (D-C+1-kyo) * kyo;
	ans += (B-A+1-kyo) * (D-C+1-kyo);
	cout<<ans<<endl;
	
	return 0;
}