#include <bits/stdc++.h>
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define vec_input(v) for(auto it=v.begin();it!=v.end();it++){cin>>*it;}
#define vec_output(v) for(auto it=v.begin();it!=v.end();it++){if(it!=v.begin())cout<<" ";cout<<*it;}cout<<endl;
#define vec_debug(v) for(auto it=v.begin();it!=v.end();it++){cerr<<*it<<" ";}cerr<<endl;
#define yn(ans) cout<<(ans?"Yes":"No")<<endl
#define YN(ans) cout<<(ans?"YES":"NO")<<endl
#define umap unordered_map
#define uset unordered_set
using namespace std;
using ll = long long;
template <typename T>T digitsum(T n);
template <typename T>bool isPrime(T n);
template <typename T>vector<pair<T, T>> prime_factor(T n);
long long int intpow(long long int,long long int);
template <typename T>T intlog(T);
long long int combination(long long int,long long int);
long long int arith_sum1(long long int,long long int,long long int);
long long int arith_sum2(long long int,long long int,long long int);
long long int series_sum(long long int);

int main(){
    int a,b,c,d,def=0;
    cin>>a>>b>>c>>d;
    if(b>=c&&b<=d)def=b-c+1;
    else if(d>=a&&d<=b)def=d-a+1;
    cout<<(b-a+1)*(d-c+1)-def<<endl;
}