import java.util.*;

public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      }
} Main public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      } main {
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A = sc.nextInt(); A sc.nextInt() sc.nextInt sc int B = sc.nextInt(); B sc.nextInt() sc.nextInt sc int C = sc.nextInt(); C sc.nextInt() sc.nextInt sc int K = sc.nextInt(); K sc.nextInt() sc.nextInt sc if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      } 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 if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      } 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 if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      } K<A+B+C K A+B+C A+B+C A B C {
    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 {
	System.out.println(A-C);	
      } System.out.println(A-C) System.out.println System.out System System.out A-C A C String[] args args public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      }
} public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int C = sc.nextInt();
    int K = sc.nextInt();
    
      if(K<=A){
    System.out.println(K);
      }else if(K<=A+B){
	System.out.println(A);
      }else if(K<A+B+C){
    System.out.println(A-(K-A-B));
      }else{
	System.out.println(A-C);	
      }
      }
}