
import java.util.Scanner;

public class Main {

	static Scanner scanner;

	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
} import java.util.Scanner; Scanner java.util public class Main {

	static Scanner scanner;

	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
} Main static Scanner scanner; scanner public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	} main {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	} scanner = new Scanner(System.in) scanner new Scanner(System.in) int A=gi(); A gi() gi int B=gi(); B gi() gi int C=gi(); C gi() gi int K=gi(); K gi() gi if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(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);
        } else {
        	System.out.print(A-(K-A-B));
        } K<A+B K A+B A B {
        	System.out.println(A);
        } System.out.println(A) System.out.println System.out System System.out A {
        	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 // 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	} gs {
		return scanner.next();
	} return scanner.next(); scanner.next() scanner.next scanner // intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	} gi {
		return Integer.parseInt(scanner.next());
	} return Integer.parseInt(scanner.next()); Integer.parseInt(scanner.next()) Integer.parseInt Integer scanner.next() scanner.next scanner // longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	} gl {
		return Long.parseLong(scanner.next());
	} return Long.parseLong(scanner.next()); Long.parseLong(scanner.next()) Long.parseLong Long scanner.next() scanner.next scanner // doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	} gd {
		return Double.parseDouble(scanner.next());
	} return Double.parseDouble(scanner.next()); Double.parseDouble(scanner.next()) Double.parseDouble Double scanner.next() scanner.next scanner public class Main {

	static Scanner scanner;

	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
} public class Main {

	static Scanner scanner;

	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
} Main

import java.util.Scanner;

public class Main {

	static Scanner scanner;

	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int A=gi();
	    int B=gi();
	    int C=gi();
	    int K=gi();


        //System.out.print(c);
        if (K<A) {
        	System.out.println(K);
        }else if(K<A+B){
        	System.out.println(A);
        } else {
        	System.out.print(A-(K-A-B));
        }
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
}