#include <iostream>
#include <set>
using namespace std;
int main(int argc, char* argv[])
{
	int A,B,C,D;
	cin>>A>>B>>C>>D;
	int i,j;
	set<pair<int,int>> mySet;
	for (i=A;i<=B;i++){
		for (j=C;j<=D;j++){
			if (i==j){
				continue;
			}
			mySet.insert(make_pair(i,j));
		}
	}
	cout<<mySet.size()<<endl;
	return 0;
}