#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(LL i = a;i<b; i++)
#define rrep(i,a,b) for(LL i = a;i>b; i--)
#define INF ((1LL<<62)-(1LL<<31))
#define ooki 9,223,372,036,854,775,807
#define tii -9,223,372,036,854,775,808
typedef long long LL;
int main(){
int a,b,c,d,count=0;
  cin >> a >> b >> c >> d;
  rep(i,a,b+1){
    rep(j,c,d+1){
      if(i!=j){count++;}
    }
  }
  cout<<count<<endl;
}