#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdint.h>
#include <iomanip>
using namespace std;

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define GETLINE( A ) string A; getline( cin , A ) 
#define FOR_ITR( ARRAY , ITR , END ) for( auto ITR = ARRAY .begin() , END = ARRAY .end() ; ITR != END ; ITR ++ ) 

int main()
{

  CIN( ll , A );
  CIN( ll , B );
  CIN( ll , C );
  CIN( ll , D );
  ll num = 0;
  
  for( ll i = A ; i <= B ; i++ ){

    for( ll j = C ; j <= D ; j++ ){

      if( i != j ){

	num++;

      }

    }

  }
  
  cout << num << endl;
  return 0;

}