import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	}

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

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	}

} Main public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	} main {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	} Scanner scan=new Scanner(System.in); scan new Scanner(System.in) int a=scan.nextInt(); a scan.nextInt() scan.nextInt scan int b=scan.nextInt(); b scan.nextInt() scan.nextInt scan int c=scan.nextInt(); c scan.nextInt() scan.nextInt scan int k=scan.nextInt(); k scan.nextInt() scan.nextInt scan if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		} k<=a k a {
			System.out.println(k);
		} System.out.println(k) System.out.println System.out System System.out k if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		} k<=a+b k a+b a b {
			System.out.println((a*1));
		} System.out.println((a*1)) System.out.println System.out System System.out (a*1) a 1 {
			System.out.println((a*1)+(-1*(k-a-b)));
		} System.out.println((a*1)+(-1*(k-a-b))) System.out.println System.out System System.out (a*1)+(-1*(k-a-b)) (a*1) a 1 (-1*(k-a-b)) -1 1 (k-a-b) k-a-b k a b String[] args args public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	}

} public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		int c=scan.nextInt();
		int k=scan.nextInt();
		if(k<=a) {
			System.out.println(k);
		}
		else if(k<=a+b) {
			System.out.println((a*1));
		}
		else {
			System.out.println((a*1)+(-1*(k-a-b)));
		}
	}

}
