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

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
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);
        BEasyLinearProgramming solver = new BEasyLinearProgramming();
        solver.solve(1, in, out);
        out.close();
    }

    static class BEasyLinearProgramming {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            long a = in.nextLong();
            long b = in.nextLong();
            long c = in.nextLong();
            long k = in.nextLong();
            if (a + b >= k) {
                if (a > k) {
                    out.println(k);
                } else {
                    out.println(a);
                }

            } else {
                long buff = k - a - b;
                out.println(a - buff);
            }

        }

    }

    static class Scanner {
        private final InputStream in;
        private final byte[] buffer;
        private int ptr;
        private int buflen;

        public Scanner(InputStream in) {
            this.in = in;
            this.buffer = new byte[1024];
            this.ptr = 0;
            this.buflen = 0;
        }

        private boolean hasNextByte() {
            if (ptr < buflen) return true;
            else {
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0) return false;
            }
            return true;
        }

        private byte readByte() {
            if (hasNextByte()) return buffer[ptr++];
            return -1;
        }

        private boolean isPrintableChar(byte c) {
            return '!' <= c && c <= '~';
        }

        private void skipUnprintable() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr])) {
                ptr++;
            }
        }

        public boolean hasNext() {
            skipUnprintable();
            return hasNextByte();
        }

        public long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            byte b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) throw new NumberFormatException();
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }

    }
}



Measured memory_bytes: 25556