import java.util.Scanner;



public class Main {


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


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


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	} main {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = Integer.parseInt(sc.next()); a Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int b = Integer.parseInt(sc.next()); b Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int c = Integer.parseInt(sc.next()); c Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int k = Integer.parseInt(sc.next()); k Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int sum = 0; sum 0 int xa = Math.min(a,k); xa Math.min(a,k) Math.min Math a k k -= xa k xa int xb = Math.min(b, k); xb Math.min(b, k) Math.min Math b k k -= xb k xb int xc = k; xc k sum = xa - xc sum xa - xc xa xc System.out.println(sum) System.out.println System.out System System.out sum String[] args args public class Main {


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	}
} public class Main {


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	}
} Main
import java.util.Scanner;



public class Main {


	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.next());
		int b = Integer.parseInt(sc.next());
		int c = Integer.parseInt(sc.next());
		int k = Integer.parseInt(sc.next());
		int sum = 0;

		int xa = Math.min(a,k);
		k -= xa;
		int xb = Math.min(b, k);
		k -= xb;
		int xc = k;
		sum = xa - xc;
		System.out.println(sum);


	}
}
