#include <bits/stdc++.h>
using namespace std;
#define int long long
signed 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++) ans += (i != j);
  cout<<ans<<endl;
}