#include <bits/stdc++.h>
using namespace std;

#define all(x) (x).begin(),(x).end()
#define SP <<" "<<
#define MOD 1000000007
#define IINF 1000000000
#define LINF 1000000000000000000

typedef long long LL;
typedef long double LD;

int main(){
  int a,b,c,d;
  cin >> a >> b >> c >> d;
  int x=max(min(b,d)-max(a,c)+1,0);
  cout << (b-a+1)*(d-c+1)-x << endl;
  return 0;
}