#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
 
using namespace std;
 
typedef long long ll;
 
#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(ll i=(a);i<(b);++i)
#define per(i,a,b) for(ll i=b-1LL;i>=(a);--i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
#define ctos(c) string(1,c)
#define endl "\n"
#define print(x) cout<<#x<<" = "<<x<<endl;
 
#define MOD 1000000007

int main(){
	ll a,b,c,d;
	cin>>a>>b>>c>>d;
	ll ans = (b-a+1)*(d-c+1);
	ll e = 0;
	rep(i,0,101){
		if(a<=i&&i<=b&&c<=i&&i<=d)e++;
	}
	cout << ans-e << endl;
	return 0;	
}