#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define fi first
#define se second
#define FOR(i,n) for(int i = 0;i<n;i++)
#define rep(i,s,n) for(int i = s;i<n;i++)
#define rrep(i,s,n) for(int i = (n)-1;i>=(s);i--)
#define all(v) (v).begin(),(v).end()
#define chmin(a,b) a=min((a),(b))
#define chmax(a,b) a=max((a),(b))
#define endl '\n'
#define IOS() ios_base::sync_with_stdio(0);cin.tie(0)
typedef long long ll;
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
const ll MOD=1000000007,INF=1ll<<60;

int A,B,C,D;
 
signed main() {
    IOS();
    cin>>A>>B>>C>>D;
    
    int ans=0;
    rep(i,A,B+1){
        rep(j,C,D+1){
            if(i!=j)ans++;
        }
    }
    cout<<ans<<endl;
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    return 0;
}