#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<math.h>
#include<complex>
#include<queue>
#include<deque>
#include<stack>
#include<map>
#include<set>
#include<bitset>
using namespace std;
#define REP(i,m,n) for(int i=(int)m ; i < (int) n ; ++i )
#define rep(i,n) REP(i,0,n)
typedef long long ll;
typedef pair<int,int> pint;
typedef pair<ll,int> pli;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;
const ll mod=1e9+7 ;

int main(){
  int a,b,c,d;
  cin >> a >> b >> c >> d;
  int ans=0;
  if(a>c){
   int temp=a;
    a=c;
    c=temp;
    int temp2=b;
    b=d;
    d=temp2;
  }
  if(b<=c){
   ans=(b-a+1)*(d-c+1); 
  }else{
   ans=(b-a+1)*(d-c+1);
    ans-=(b-c+1);
  }
  cout << ans << endl;
return 0;}