#include<bits/stdc++.h>
using namespace std;

#define IOS std::cin.tie(0);std::ios_base::sync_with_stdio(false)

#define ll      long long
#define Fi      first
#define Se      second
#define Pb      push_back
#define vi      vector<int>
#define vp      vector<pair<int,int>>
#define vvi     vector<vector<int>>
#define vvp     vector<vector<pair<int,int>>>

#define sz(x) (int)x.size()
#define all(v) v.begin(),v.end()
#define output_s(s) printf("%s\n",(s).c_str())
#define END cout<<"\n"

#define rep(i,n)        for(int i=0;i<(int)(n);i++)
#define Rep(i,n,m)      for(int i=(int)(n);i<(int)(m);i++)
#define RepS(i,n,m)     for(int i=(int)(n);i<=(int)(m);i++)
#define RRepS(i,n,m)    for(int i=(int)(n);i>=(int)(m);i--)
#define SRepS(i,n,m)    for(int i=(int)(n);i*i<=(int)(m);i++)
#define RepA(x)         for(auto &item:(x))

void CINT(){} //@shibh308's template
template<class Head,class... Tail> void CINT(Head&& head,Tail&&... tail){cin>>head; CINT(move(tail)...);}    
#define CIN(...) int __VA_ARGS__;CINT(__VA_ARGS__)
#define LCIN(...) ll __VA_ARGS__;CINT(__VA_ARGS__)
#define SCIN(...) string __VA_ARGS__;CINT(__VA_ARGS__)
#define VCIN(x) for(auto &item:x) cin>>item

void OUT(){}
template<class Head,class...Tail> void OUT(Head&& head,Tail&&...tail){cout<<head<<(sizeof...(tail)?" ":"\n"); OUT(move(tail)...);}
#define COUT(...) OUT(__VA_ARGS__)

template<typename TypeInt>TypeInt GCD(TypeInt a,TypeInt b){return a%b==0?b:GCD(b,a%b);} //Greatest Common Divisor
template<typename TypeInt>TypeInt LCM(TypeInt a,TypeInt b){return a*b/GCD(a,b);} //Least Common Multiple
template<class T>inline bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>inline bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}
template<class T>bool comp(T a,T b){return abs(a)<abs(b);} //比較関数

void YES(bool f){printf(f?"YES\n":"NO\n");}
void Yes(bool f){printf(f?"Yes\n":"No\n");}
void Possible(bool f){printf(f?"Possible\n":"Impossible\n");}
void POSSIBLE(bool f){printf(f?"POSSIBLE\n":"IMPOSSIBLE\n");}

//---------------------Template------------------------//

//Prime Factorization
//The result is stored in the array
void PrimeFactorize(long long num,vector<long long> &ret){
    vector<long long> prime(num+1,0);
    vector<long long>::iterator it;
    while(num%2==0){
        num /= 2;
        prime[2]++;
    }
    for(long long i=3;i<=num;i++){
        while(num%i==0){
            num /= i;
            prime[i]++;
        }
    }
    copy(prime.begin(),prime.end(),back_inserter(ret));
}

//Judge the prime number
bool IsPrime(int x){
    if(x<2) return false;
    else if(x==2) return true;
    else if(x%2==0) return false;
    for(int i=3;i*i<=x;i+=2){
        if(x%i==0)return false;
    }
    return true;
}

//Meguru binary search
//left < right
template<typename T>class BinarySearch{
    T _l;
    T _r;
    function<bool(T)> _f;
    public:
        BinarySearch(T left,T right,function<bool(T)>func){
            this->_l=left;
            this->_r=right;
            this->_f=func;
        }
        ~BinarySearch() = default;
        T calc(){
            while((this->_r-this->_l)>1){
                T middle=(this->_l+this->_r)/2;
                if(this->_f(middle)){
                    this->_r=middle;
                }else{
                    this->_l=middle;
                }
            }
            return this->_r;
        }
};

//Dijkstra
//範囲外エラーが起きるので使用不可
template<typename T>class Dijkstra{
    T v; //vertex
    struct edge{
        T to;
        T cost;
    };
    const T INF=1e9;
 
    public:
    vector<vector<edge>> Graph;
    vector<T> dist;
 
    Dijkstra(T _v):Graph(v),dist(v,INF){
        this->v=_v;
    };
    virtual ~Dijkstra(){
        vector<vector<edge>>().swap(Graph);
    };
    /**
     * @param _f    from
     * @param _t    to
     * @param _c    cost (default = 1)
    */
    void addEdge(T _f, T _t, T _c = 1){
        Graph[_f].push_back((edge){_t,_c});
    }
    /**
     * @param _f Start position
     * @param _t Destination
    */
    T shortestPath(T _f,T _t){
        priority_queue<pair<T,T>,vector<pair<T,T>>,greater<pair<T,T>>> que;
 
        que.push(make_pair(0,_f));
        dist[_f]=0;
 
        while(!que.empty()){
            T v=que.top().second;
            que.pop();
            
            for(int i=0;i<Graph[v].size();i++){
                edge e=Graph[v][i];
                if(dist[e.to]>dist[v]+e.cost){
                    dist[e.to]=dist[v]+e.cost;
                    que.push(make_pair(dist[e.to],e.to));
                }
            }
        }
        return dist[_t];
    }
};

//----------------------Library------------------------//

const int dx4[]={1,0,-1,0}, dx8[]={1,0,-1,0,1,-1,-1,1};
const int dy4[]={0,1,0,-1}, dy8[]={0,1,0,-1,1,1,-1,-1};
const int NMAX = 55;
const int INF = 1e9;
const long long LINF = 1e18;
const double EPS = 1e-10;
const int MOD = 1e9+7;
bool flag = false;

//----------------------Global-------------------------//

signed main(){
    CIN(a,b,c,k);
    int ans=0;
    if(k>=0){
        ans+=min(a,k);
        k-=min(a,k);
    }
    if(k>=0)k-=min(b,k);
    if(k>=0){
        ans-=min(c,k);
        k-=min(c,k);
    }
    COUT(ans);
    
    return 0;
}

Measured memory_bytes: 3600