#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll inf = 100100100100100;
const ll mod = 1000000007;


int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
   int a,b,c,d;
   cin>>a>>b>>c>>d;
   ll count=0;
   for(int i=a;i<=b;i++){
       for(int j=c;j<=d;j++){
           if(i==j)continue;
           count++;
       }
   }
cout<<count<<endl;
    
}