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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		}
	}

} 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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(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 sc.close() sc.close sc if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		} (k - a) <= 0 (k - a) k a 0 {
			System.out.print(k);
		} System.out.print(k) System.out.print System.out System System.out k if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		} (k - (a + b)) <= 0 (k - (a + b)) k (a + b) a b 0 {
			System.out.print(a);
		} System.out.print(a) System.out.print System.out System System.out a if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		} (k - (a + b + c)) <= 0 (k - (a + b + c)) k (a + b + c) a + b + c a b c 0 {
			System.out.print(a - (k - a - b));
		} System.out.print(a - (k - a - b)) System.out.print 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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		}
	}

} 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();
		sc.close();

		if ((k - a) <= 0) {
			System.out.print(k);
		} else if ((k - (a + b)) <= 0) {
			System.out.print(a);
		} else if ((k - (a + b + c)) <= 0) {
			System.out.print(a - (k - a - b));
		}
	}

}
