import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    }

} Main public static void main(String[] args) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    } main {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    } @SuppressWarnings("resource")
        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);
            return;
        } k <= a k a {
            System.out.println(k);
            return;
        } System.out.println(k) System.out.println System.out System System.out k return; if (k <= a + b) {
            System.out.println(a);
            return;
        } k <= a + b k a + b a b {
            System.out.println(a);
            return;
        } System.out.println(a) System.out.println System.out System System.out a return; 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) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    }

} public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        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);
            return;
        }
        if (k <= a + b) {
            System.out.println(a);
            return;
        }
        System.out.println(a - (k - a - b));
    }

}