import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a=sc.nextLong(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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 sc = new Scanner(System.in);
    long a=sc.nextLong(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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 b=sc.nextLong() b sc.nextLong() sc.nextLong sc c=sc.nextLong() c sc.nextLong() sc.nextLong sc k=sc.nextLong(); k sc.nextLong() sc.nextLong sc long res=0; res 0 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 sc = new Scanner(System.in);
    long a=sc.nextLong(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    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 sc = new Scanner(System.in);
    long a=sc.nextLong(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    if(k<=a) {
    	System.out.println(k);
    }else if(k<=a+b){
    	System.out.println(a);
    }else {
    	System.out.println(a-(k-a-b));
    }
  }
}
