#include<iostream>
using namespace std;

int main(){
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	int ans=0;
	for(int i=a;i<=b;++i)for(int j=c;j<=d;++j){
		if(i!=j)++ans;
	}
	cout<<ans<<endl;
	return 0;
}