#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define P pair<ll,ll>
#define FOR(I,A,B) for(ll I = ll(A); I < ll(B); ++I)
#define FORR(I,A,B) for(ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x,t,f) ((x)?(t):(f))
#define SORT(x) (sort(x.begin(),x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin()) //xi>=v  x is sorted
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin()) //xi>v  x is sorted
#define NUM(x,v) (POSU(x,v)-POSL(x,v))  //x is sorted
#define REV(x) (reverse(x.begin(),x.end())) //reverse
ll gcd(ll a,ll b){if(a<b)swap(a,b);if(a%b==0)return b;return gcd(b,a%b);}
ll lcm(ll a,ll b){ll c=gcd(a,b);return ((a/c)*(b/c)*c);}//saisyo kobaisu
#define NEXTP(x) next_permutation(x.begin(),x.end())
const ll INF=1e18+7;
const ll MOD=1e9+7;
#define out(a) cout<<fixed<<setprecision((a))



int main(){
	ll a,b,c,d,ans=0;
	cin >> a >> b >> c >> d;
	FOR(i,a,b+1){
		if(i<=d && i>=c){
			ans += d-c;
		}else{
			ans += d-c+1;
		}
	}
	cout << ans << endl;
}