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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  }
} 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  }
} 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  } 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  } 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    } a >= k a k {
      System.out.println(k);
    } System.out.println(k) System.out.println System.out System System.out k if(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    } a < k && a + b >= k a < k a k a + b >= k a + b a b k {
      System.out.println(a);
    } System.out.println(a) System.out.println System.out System System.out a if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    } a + b < k && a + b+ c >= k a + b < k a + b a b k a + b+ c >= k a + b+ c a + b+ c a b c k {
      System.out.println(2 * a + b - k);
    } System.out.println(2 * a + b - k) System.out.println System.out System System.out 2 * a + b - k 2 * a + b 2 * a 2 a b k 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  }
} 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  }
} 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(a < k && a + b >= k) {
      System.out.println(a);
    }else if(a + b < k && a + b+ c >= k) {
      System.out.println(2 * a + b - k);
    }
  }
}