#include<bits/stdc++.h>
using namespace std;

int main(){
	cin.tie(0),ios::sync_with_stdio(false);
	int a,b,c,d; cin>>a>>b>>c>>d;
	int diff=min(b,d)-max(a,c)+1;
	cout<<(diff<0?(b-a+1)*(d-c+1):abs((b-a+1-diff)*(d-c+1))+diff*(d-c))<<"\n"s;
}