#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define VIN(v) for(auto&elem_: (v) )cin>>elem_
#define VOUT(v, sep) for(_loop_int idx=0; idx<(_loop_int)v.size(); idx++) {cout<<v[idx]; if(idx<v.size()-1) cout<<sep;} cout<<endl

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG2(x,y) cout<<#x<<": "<<x<<" "<<#y<<": "<<y<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(debugidx,v.size())cout<<" "<<v[debugidx];cout<<endl
#define DEBUG_ARR(v,n) cout<<#v<<":";REP(debugidx,n)cout<<" "<<v[debugidx];cout<<endl
#define ALL(a) (a).begin(),(a).end()

const ll MOD = 1000000007ll;
const int IINF = numeric_limits<int>::max()/2-1;
const ll LINF = numeric_limits<ll>::max()/2-1;
#define FIX(a) ((a)%MOD+MOD)%MOD

int a,b,c,d;
int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    cin>>a>>b>>c>>d;

    int ans=0;
    FOR(i,a,b+1){
        if(i>=c && i<=d){
            ans+=d-c;
        }else{
            ans+=d-c+1;
        }
    }
    cout << ans << endl;
    return 0;
}