import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Mehvix
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) abc167_b solver = new abc167_b(); solver new abc167_b() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    } abc167_b public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        } solve {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        } int a = fin.nextInt(); a fin.nextInt() fin.nextInt fin int b = fin.nextInt(); b fin.nextInt() fin.nextInt fin int c = fin.nextInt(); c fin.nextInt() fin.nextInt fin int k = fin.nextInt(); k fin.nextInt() fin.nextInt fin int sum = 0; sum 0 sum += Math.min(a, Math.max(k, 0)) sum Math.min(a, Math.max(k, 0)) Math.min Math a Math.max(k, 0) Math.max Math k 0 k -= a k a k -= b k b sum -= Math.min(c, Math.max(k, 0)) sum Math.min(c, Math.max(k, 0)) Math.min Math c Math.max(k, 0) Math.max Math k 0 fout.println(sum) fout.println fout sum int testNumber testNumber Scanner fin fin PrintWriter fout fout public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Mehvix
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        abc167_b solver = new abc167_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class abc167_b {
        public void solve(int testNumber, Scanner fin, PrintWriter fout) {
            int a = fin.nextInt();
            int b = fin.nextInt();
            int c = fin.nextInt();
            int k = fin.nextInt();

            int sum = 0;

            sum += Math.min(a, Math.max(k, 0));
            k -= a;
            k -= b;
            sum -= Math.min(c, Math.max(k, 0));

            fout.println(sum);
        }

    }
}

