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