#include <bits/stdc++.h>
#define inf 1000000000 //1E+9
using namespace std;
 
int main(){
	int a,c,b,d;cin>>a>>b>>c>>d;
	int ans=(b-a+1)*(d-c+1);
	set<int> s;
	for(int i=a;i<=b;i++)s.insert(i);
	for(int i=c;i<=d;i++)if(s.count(i))ans--;

	cout<<ans<<endl;
 
	return 0;
}