import java.io.*;
import java.util.*;
/**
 * Main
 */
public class Main {
       static int mod  = 1000000007;
        static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    }public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
} import java.io.*; io.* java import java.util.*; util.* java public class Main {
       static int mod  = 1000000007;
        static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    }public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
} Main static int mod  = 1000000007; mod 1000000007 static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out)); pw new PrintWriter(new OutputStreamWriter(System.out)) public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    } main {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    } MyScanner ms  = new MyScanner(); ms new MyScanner() long A = ms.nextLong(); A ms.nextLong() ms.nextLong ms long B = ms.nextLong(); B ms.nextLong() ms.nextLong ms long C = ms.nextLong(); C ms.nextLong() ms.nextLong ms long K =ms.nextLong(); K ms.nextLong() ms.nextLong ms long sum=0; sum 0 if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       } K>0 K 0 {
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       } if(A>K)
        sum+=K;
        else
        sum+=A; A>K A K sum+=K sum K sum+=A sum A K-=A K A if(K>0){
           K-=B;
       } K>0 K 0 {
           K-=B;
       } K-=B K B if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       } K>0 K 0 {
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       } if(C>K)
        sum-=K;
        else
        sum-=C; C>K C K sum-=K sum K sum-=C sum C K-=C K C pw.println(sum) pw.println pw sum pw.flush() pw.flush pw String[] args args public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     } MyScanner BufferedReader br; br StringTokenizer st; st public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        } MyScanner {
           br = new BufferedReader(new InputStreamReader(System.in));
        } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        } next {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        } while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } IOException e {
                    e.printStackTrace();
                } e.printStackTrace() e.printStackTrace e {
                    st = new StringTokenizer(br.readLine());
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() {
            return Integer.parseInt(next());
        } nextInt {
            return Integer.parseInt(next());
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() {
            return Long.parseLong(next());
        } nextLong {
            return Long.parseLong(next());
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() {
            return Double.parseDouble(next());
        } nextDouble {
            return Double.parseDouble(next());
        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        } nextLine {
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        } String str = ""; str "" try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        } catch (IOException e) {
           e.printStackTrace();
        } IOException e {
           e.printStackTrace();
        } e.printStackTrace() e.printStackTrace e {
           str = br.readLine();
        } str = br.readLine() str br.readLine() br.readLine br return str; str public class Main {
       static int mod  = 1000000007;
        static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    }public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
} public class Main {
       static int mod  = 1000000007;
        static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    }public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
} Main
import java.io.*;
import java.util.*;
/**
 * Main
 */
public class Main {
       static int mod  = 1000000007;
        static PrintWriter pw = new PrintWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) {
       MyScanner ms  = new MyScanner();
       long A = ms.nextLong();
       long B = ms.nextLong();
       long C = ms.nextLong();
       long K =ms.nextLong();
       long sum=0;
       if(K>0){
        if(A>K)
        sum+=K;
        else
        sum+=A;
        K-=A;
       }
       if(K>0){
           K-=B;
       }
       if(K>0){
        if(C>K)
        sum-=K;
        else
        sum-=C;
           K-=C;
       }
       pw.println(sum);
    pw.flush();

    }public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
}