import java.util.Scanner;
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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    }
} 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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    }
} 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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    } main {
    Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        int K = sc.nextInt();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    } 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 if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B)); K<=A K A System.out.println(K) System.out.println System.out System System.out K if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B)); K <= A+B K A+B A B System.out.println(A) System.out.println System.out System System.out A System.out.println(A-(K-A-B)) System.out.println System.out System System.out A-(K-A-B) A (K-A-B) K-A-B K A B 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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    }
} 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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    }
} Main
import java.util.Scanner;
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();
    
    	if(K<=A) System.out.println(K);
    	else if(K <= A+B) System.out.println(A);
    	else System.out.println(A-(K-A-B));
    }
}