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(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          System.out.println(a);
        }else{
          System.out.println(a-(k-a-b));
        }
    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          System.out.println(a);
        }else{
          System.out.println(a-(k-a-b));
        }
    } 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(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          System.out.println(a);
        }else{
          System.out.println(a-(k-a-b));
        } a >= k a k {
          System.out.println(k);
        } System.out.println(k) System.out.println System.out System System.out k if(k-a-b < 0){
          System.out.println(a);
        }else{
          System.out.println(a-(k-a-b));
        } k-a-b < 0 k-a-b k-a-b k a b 0 {
          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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        if(a >= k){
          System.out.println(k);
        }else if(k-a-b < 0){
          System.out.println(a);
        }else{
          System.out.println(a-(k-a-b));
        }
    }
}
