#include <bits/stdc++.h>
#define mp make_pair
#define all(vec) vec.begin(),vec.end()
using namespace std;
using ll=long long;
using P=pair<ll,ll>;
const ll INF=1LL<<30;
const ll LINF=1LL<<60;
const double eps=1e-9;
const ll MOD=1000000007LL;
template<typename T>void chmin(T &a,T b){a=min(a,b);};
template<typename T>void chmax(T &a,T b){a=max(a,b);};
template<typename T>vector<T> make_v(size_t a){return vector<T>(a);}
template<typename T,typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));}
template<typename T,typename V>typename enable_if<is_class<T>::value==0>::type fill_v(T& t,const V& v){t=v;}
template<typename T,typename V>typename enable_if<is_class<T>::value!=0>::type fill_v(T& t,const V& v){for(auto &e:t)fill_v(e,v);};
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
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;
}