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