#pragma GCC optimize("Ofast","unroll-loops")
//#pragma GCC push_options
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
#include <xmmintrin.h>
#include <immintrin.h>
using namespace::std;
__attribute__((constructor))void init(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
typedef mp::number<mp::cpp_dec_float<0>> cdouble;
typedef mp::cpp_int cint;
template<typename T>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T,typename E>using hash_map=__gnu_pbds::gp_hash_table<T,E>;
template<typename T>using pqueue =__gnu_pbds::priority_queue<T, less<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define MOD 1000000007LL
//#define MOD 998244353LL
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(lint i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(lint i=0;i<t.size();i++)output(t[i]);}
#define rep(i,n) for(lint i=0;i<lint(n);++i)
#define repi(i,a,b) for(lint i=lint(a);i<(lint)(b);++i)
#define rrep(i,n) for(lint i=lint(n)-1;i>=0;--i)
#define rrepi(i,a,b) for(lint i=lint(b)-1;i>=lint(a);--i)
#define irep(i) for(lint i=0;;++i)
#define all(n) begin(n),end(n)
#define dist(a,b,c,d) sqrt(pow(a-c,2)+pow(b-d,2))
inline lint gcd(lint A,lint B){return B?gcd(B,A%B):A;}
inline lint lcm(lint A,lint B){return A/gcd(A,B)*B;}
// inline cint cgcd(cint A,cint B){return B?cgcd(B,A%B):A;}
// inline cint clcm(cint A,cint B){return A/cgcd(A,B)*B;}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
const vector<lint> dx={1,0,-1,0,1,1,-1,-1};
const vector<lint> dy={0,1,0,-1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
auto call=[&](auto f,auto... args){return f(f,args...);};

class mint {
  using u64 = std::uint_fast64_t;
    public:
    u64 a;
    constexpr mint(const u64 x = 0)noexcept:a(x % MOD){}
    constexpr u64 &value()noexcept{return a;}
    constexpr const u64 &value() const noexcept {return a;}
    constexpr mint operator+(const mint rhs)const noexcept{return mint(*this) += rhs;}
    constexpr mint operator-(const mint rhs)const noexcept{return mint(*this)-=rhs;}
    constexpr mint operator*(const mint rhs) const noexcept {return mint(*this) *= rhs;}
    constexpr mint operator/(const mint rhs) const noexcept {return mint(*this) /= rhs;}
    constexpr mint &operator+=(const mint rhs) noexcept {
        a += rhs.a;
        if (a >= MOD)a -= MOD;
        return *this;
    }
    constexpr mint &operator-=(const mint rhs) noexcept {
        if (a<rhs.a)a += MOD;
        a -= rhs.a;
        return *this;
    }
    constexpr mint &operator*=(const mint rhs) noexcept {
        a = a * rhs.a % MOD;
        return *this;
    }
    constexpr mint operator++(int n) noexcept {
        a += 1;
        if (a >= MOD)a -= MOD;
        return *this;
    }
    constexpr mint operator--(int n) noexcept {
        if (a<1)a += MOD;
        a -= 1;
        return *this;
    }
    constexpr mint &operator/=(mint rhs) noexcept {
        u64 exp=MOD-2;
        while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
        }
        return *this;
    }
    mint pow(long long n){
        long long x=a;
        mint ret = 1;
        while(n>0) {
            if(n&1)(ret*=x);
            (x*=x)%=MOD;
            n>>=1;
        }
        return ret;
    }
    mint inv()const{
        int _a=a,b=MOD,u=1,v=0,t;
        while(b>0) {
            t=_a/b;
            swap(_a-=t*b,b);
            swap(u-=t*v,v);
        }
        return mint(u);
    }
    mint comb(lint b){
        using lint=long long;
        static bool init=1;
        static lint fac[1000001],ifac[1000001];
        if(init){
            init=0;
            fac[0]=1;
            ifac[0]=1;
            auto mod_pow=[&](lint x,lint n){
                lint ans = 1;
                while(n != 0){
                    if(n&1)ans=ans*x%MOD;
                    x=x*x%MOD;
                    n=n>>1;
                }
                return ans;
            };
            for(int i=0;i<1000000;i++){
                fac[i+1]=fac[i]*(i+1)%MOD;
                ifac[i+1]=ifac[i]*mod_pow(i+1, MOD-2)%MOD;
            }
        }
        if(a==0&&b==0)return 1;
        if((lint)a<b||a<0)return 0;
        lint tmp=ifac[a-b]*ifac[b]%MOD;
        return tmp*fac[a]%MOD;
    }
    
    friend ostream& operator<<(ostream& lhs, const mint& rhs) noexcept {
        lhs << rhs.a;
        return lhs;
    }
    friend istream& operator>>(istream& lhs,mint& rhs) noexcept {
        lhs >> rhs.a;
        return lhs;
    }
};
inline mint fact(lint a){return a?fact(a-1)*a:1;}

template<typename T>
class segment{
	T* node;
	int n=1;
	public:
	segment(int sz){
		while(n<=sz)n<<=1;
		node=new T[(n<<1)-1];
		for(int i=0;i<(n<<1)-1;i++)node[i]=e;
	}
	segment(const vector<T>& v){
		while(n<(int)v.size())n<<=1;
		node=new T[(n<<1)-1];
		for(int i=0;i<(int)v.size();i++)node[i+n-1]=v[i];
		for(int i=n-2;i>=0;i--)node[i]=f(node[(i<<1)+1],node[(i<<1)+2]);
	}
	T get(auto l,auto r){
		l+=n;r+=n;
		T s=e;
		while(l<r){
			if(l&1)s=f(s,node[l++-1]);
			if(r&1)s=f(s,node[--r-1]);
			l>>=1;r>>=1;
		}
		return s;
	}
	void update(auto t,T val){
		t+=n-1;
		while(1){
			node[t]=f(node[t],val);
			t=(t-1)>>1;
			if(!t)break;
		}
	}
	constexpr static T e=0;
	constexpr static
	T f(const T& s,const T& t){
		return s+t;
	}
};

auto lis(auto v){
    decltype(v) res;
    for(int i=0;i<(int)v.size();++i){
        auto itr=lower_bound(all(res),v[i]);
        if(itr==res.end())res.push_back(v[i]);
        else res[itr-res.begin()]=v[i];
    }
    return res;
}



int main(){
    lint a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<max(0LL,min(b,d)-max(a,c))<<endl;
}


Measured memory_bytes: 256