#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
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++)
      if(i!=j) ans++;
  cout<<ans<<endl;
  return 0;
}