Language: Java

import java.util.*;
class Main{import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

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

    static class ADoubleHelix {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            char s = in.nc();
            if (s == 'A')
                out.println("T");
            else if (s == 'T')
                out.println("A");
            else if (s == 'C')
                out.println("G");
            else
                out.println("C");
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public char nc() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            return (char) c;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }
}



Compiler output:
Main.java:2: error: illegal start of type
class Main{import java.io.OutputStream;
           ^
Main.java:2: error: <identifier> expected
class Main{import java.io.OutputStream;
                                      ^
Main.java:3: error: illegal start of type
import java.io.IOException;
^
Main.java:3: error: <identifier> expected
import java.io.IOException;
                          ^
Main.java:4: error: illegal start of type
import java.io.InputStream;
^
Main.java:4: error: <identifier> expected
import java.io.InputStream;
                          ^
Main.java:5: error: illegal start of type
import java.io.OutputStream;
^
Main.java:5: error: <identifier> expected
import java.io.OutputStream;
                           ^
Main.java:6: error: illegal start of type
import java.io.PrintWriter;
^
Main.java:6: error: <identifier> expected
import java.io.PrintWriter;
                          ^
Main.java:7: error: illegal start of type
import java.io.BufferedWriter;
^
Main.java:7: error: <identifier> expected
import java.io.BufferedWriter;
                             ^
Main.java:8: error: illegal start of type
import java.io.Writer;
^
Main.java:8: error: <identifier> expected
import java.io.Writer;
                     ^
Main.java:9: error: illegal start of type
import java.io.OutputStreamWriter;
^
Main.java:9: error: <identifier> expected
import java.io.OutputStreamWriter;
                                 ^
Main.java:10: error: illegal start of type
import java.util.InputMismatchException;
^
Main.java:10: error: <identifier> expected
import java.util.InputMismatchException;
                                       ^
Main.java:11: error: illegal start of type
import java.io.IOException;
^
Main.java:11: error: <identifier> expected
import java.io.IOException;
                          ^
Main.java:12: error: illegal start of type
import java.io.InputStream;
^
Main.java:12: error: <identifier> expected
import java.io.InputStream;
                          ^
Main.java:131: error: reached end of file while parsing
}
 ^
23 errors