import java.util.Scanner;

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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  }
} import java.util.Scanner; Scanner 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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  }
} 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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  } main {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  } 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 int answer = 0; answer 0 if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    } k > a + b k a + b a b {
      answer = a - (k - a - b);
    } answer = a - (k - a - b) answer a - (k - a - b) a (k - a - b) k - a - b k a b {
      answer = Math.min(a,k);
    } answer = Math.min(a,k) answer Math.min(a,k) Math.min Math a k System.out.println(answer) System.out.println System.out System System.out answer 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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  }
} 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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  }
} Main
import java.util.Scanner;

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();
    int answer = 0;
    if(k > a + b){
      answer = a - (k - a - b);
    } else {
      answer = Math.min(a,k);
    }
    System.out.println(answer);
  }
}