#include <iostream>
#include <vector>
#include <map>
#include<string>
#define ll long long
using namespace std;
int main() {
    int a,b,c,d,ans;
    cin>>a>>b>>c>>d;
    ans=(b-a+1)*(d-c+1)-max(0,min(b,d)-max(a,c)+1);
    cout<<ans<<endl;
    return 0;
}