
import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       }
	}
} import java.util.*; util.* java public class Main {// Main
	public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       }
	}
} Main public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }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+b ) {
    	   System.out.println(Math.min(a, k));
       }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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       } k <= a+b k a+b a b {
    	   System.out.println(Math.min(a, k));
       } System.out.println(Math.min(a, k)) System.out.println System.out System System.out Math.min(a, k) Math.min Math a k {
    	   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 {// Main
	public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       }
	}
} public class Main {// Main
	public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       }
	}
} Main

import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		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+b ) {
    	   System.out.println(Math.min(a, k));
       }else {
    	   System.out.println(a - (k-a-b));
       }
	}
}