#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define INF (1<<30)
#define INFLL (1ll<<60)
typedef pair<double, int> P;
typedef pair<int, P> E;
#define MOD (1000000007ll)
#define l_ength size
#define EPS (1e-10)

void add_mod(ll &a, ll b){
	a += b;
	a %= MOD;
}

void mul_mod(ll &a, ll b){
	a *= b;
	a %= MOD;
}

ll cnt[8004004];

int main(void){
	int a,b,c,d;
	cin >> a >> b >> c >> d;
	cout << ((b-a+1)*(d-c+1)-max(min(b,d)-max(a,c)+1,0)) << endl;
	return 0;
}