#include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <numeric>
#include <cmath>

using namespace std;

typedef long long int ll;

#define all(x) x.begin(),x.end()

const ll mod = 1e9+7;
const ll INF = 1e9;
const ll MAXN = 1e9;

int main()
{
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	int cnt=0;
	for(int i = 1; i <= 100; i++){
		if(a<=i&&i<=b && c<=i&&i<=d)cnt++;
	}
	cout << (b-a+1)*(d-c+1) - cnt << endl;

	return 0;
}