#include <iostream>
#include <iomanip>
#include <vector>
#include <utility>
#include <map>
#include <algorithm>
#include <queue>
#include <cmath>
#include <numeric>
#include <set>

using namespace std;
struct aaa{aaa(){cin.tie(nullptr); ios::sync_with_stdio(false); cout<<fixed<<setprecision(20);};}aaa;
template <class T>ostream &operator<<(ostream &o,const vector<T>&v){o<<"{";for(int i=0;i<(int)v.size();i++)o<<(i>0?", ":"")<<v[i];o<<"}";return o;}
#define debug(v) {cerr<<"\033[1;36m[debug]\033[m "<<#v<<" : "<<v<<endl;}

template<typename T>bool chmax(T a,T& b) {if (b<a) {b=a;return true;} return false;}

using int64 = long long;

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;
}