import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  }
} import java.util.Scanner; Scanner java.util public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  }
} Main public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  } main {
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = scan.nextInt(); a scan.nextInt() scan.nextInt scan int b = scan.nextInt(); b scan.nextInt() scan.nextInt scan int c = scan.nextInt(); c scan.nextInt() scan.nextInt scan int k = scan.nextInt(); k scan.nextInt() scan.nextInt scan if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    } k <= a k a {
      System.out.println(k);
    } System.out.println(k) System.out.println System.out System System.out k if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    } k <= a+b k a+b a b {
      System.out.println(a);
    } System.out.println(a) System.out.println System.out System System.out a {
      System.out.println(a-(k-a-b));
    } System.out.println(a-(k-a-b)) System.out.println System.out System System.out a-(k-a-b) a (k-a-b) k-a-b k a b String[] args args public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  }
} public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  }
} Main
import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    if (k <= a){
      System.out.println(k);
    }else if (k <= a+b){
      System.out.println(a);
    }else{
      System.out.println(a-(k-a-b));
    }
  }
}