#include <bits/stdc++.h>
#define int long long int
using namespace std;
template<typename T,typename U> using P=pair<T,U>;
template<typename T> using V=vector<T>;
template<typename T>bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<typename T>bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}

template<typename T>auto&operator<<(ostream&s,const vector<T>&v){s<<"[";bool a=1;for(auto e:v){s<<(a?"":" ")<<e;a=0;}s<<"]";return s;}
template<typename T,typename U>auto&operator<<(ostream&s,const pair<T,U>&p){s<<"("<<p.first<<","<<p.second<<")";return s;}
template<typename T>auto&operator<<(ostream&s,const set<T>&st){s<<"{";bool a=1;for(auto e:st){s<<(a?"":" ")<<e;a=0;}s<<"}";return s;}
template<typename T,typename U>auto&operator<<(ostream&s,const map<T,U>&m){s<<"{";bool a=1;for(auto e:m){s<<(a?"":" ")<<e.first<<":"<<e.second;a=0;}s<<"}";return s;}
#define DUMP(x)  cerr<<#x<<" = "<<(x)<<endl;

struct edge { int to, cost; };

const int INF = 1e18;
const int MOD = 1e9+7;

signed main()
{
   int a, b, c, d; cin >> a >> b >> c >> d;

   int ans = (b - a + 1) * (d - c + 1) - max(min(b,d) - max(a,c) + 1, 0LL);
   cout << ans << endl;

   return 0;
}