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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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 for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		} int i = 0; int i = 0; i 0 i < k i k i++ i++ i {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		} {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		} if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			} a > 0 a 0 {
				sum += 1;
				a -= 1;
			} sum += 1 sum 1 a -= 1 a 1 if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			} b > 0 b 0 {
				b -= 1;
			} b -= 1 b 1 if(c > 0){
				sum -= 1;
				c -= 1;
			} c > 0 c 0 {
				sum -= 1;
				c -= 1;
			} sum -= 1 sum 1 c -= 1 c 1 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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		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;
		for(int i = 0; i < k; i++) {
			if(a > 0) {
				sum += 1;
				a -= 1;
			}else if(b > 0) {
				b -= 1;
			}else if(c > 0){
				sum -= 1;
				c -= 1;
			}
		}
		System.out.println(sum);

	}
}
