import java.util.Scanner;

//public class Begginer05 {
	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();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 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 = sc.nextInt();
		int B = sc.nextInt();
		int C = sc.nextInt();
		int K = sc.nextInt();
		sc.close();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	}
} 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();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	} 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();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	} 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 int sum = 0; sum 0 for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 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 (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		} {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		} if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			} A != 0 A 0 {
				sum += 1;
				A -= 1;
			} sum += 1 sum 1 A -= 1 A 1 if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			} A == 0 && B != 0 A == 0 A 0 B != 0 B 0 {
				B -= 1;
			} B -= 1 B 1 {
				sum -= 1;
			} sum -= 1 sum 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 = sc.nextInt();
		int B = sc.nextInt();
		int C = sc.nextInt();
		int K = sc.nextInt();
		sc.close();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	}
} 	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();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	}
} Main
import java.util.Scanner;

//public class Begginer05 {
	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();

		int sum = 0;
		
		for (int i = 0; i < K ; i++) {
			if (A != 0) {
				sum += 1;
				A -= 1;
			} else if (A == 0 && B != 0) {
				B -= 1;
			} else {
				sum -= 1;
			}
		}
		System.out.println(sum);
	}
}
