import java.util.*;

public class Main {
    public static void main(String[] args ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        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.*; util.* java public class Main {
    public static void main(String[] args ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        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 ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        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();
        long K = sc.nextLong();
      
        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 long K = sc.nextLong(); K sc.nextLong() sc.nextLong 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(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(A) System.out.println System.out System System.out A {
            System.out.println(A-(K-A-B));
        } 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 ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        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 ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        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.*;

public class Main {
    public static void main(String[] args ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        long K = sc.nextLong();
      
        if(K <= A) {
            System.out.println(K);
        } else if(K < A+B) {
            System.out.println(A);
        } else {
            System.out.println(A-(K-A-B));
        }

    }
}
