source_code: 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 lewin
   */
  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);
          BTheHat solver = new BTheHat();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class BTheHat {
          InputReader in;
          OutputWriter out;
          int n;
  
          int ask(int student) {
              student %= n;
              out.println("? " + (student + 1));
              out.flush();
              return in.nextInt();
          }
  
          public void solve(int testNumber, InputReader in, OutputWriter out) {
              this.in = in;
              this.out = out;
  
              n = in.nextInt();
              int a = ask(0), b = ask(n / 2);
              if ((a + b) % 2 != 0) {
                  out.println("! -1");
                  out.flush();
                  return;
              }
              if (a == b) {
                  out.println("! 1");
                  out.flush();
                  return;
              }
  
              int lo = 0, hi = n / 2;
              while (lo < hi) {
                  int mid = (lo + hi) / 2;
                  int f1 = ask(mid), f2 = ask(mid + n / 2);
                  if (f1 == f2) {
                      out.println("! " + (mid + 1));
                      return;
                  }
                  if ((a > b) == (f1 > f2)) {
                      lo = mid + 1;
                  } else {
                      hi = mid - 1;
                  }
              }
              out.println("! " + (lo + 1));
              out.flush();
          }
  
      }
  
      static class InputReader {
          private InputStream stream;
          private byte[] buf = new byte[1 << 16];
          private int curChar;
          private int numChars;
  
          public InputReader(InputStream stream) {
              this.stream = stream;
          }
  
          public int read() {
              if (this.numChars == -1) {
                  throw new InputMismatchException();
              } else {
                  if (this.curChar >= this.numChars) {
                      this.curChar = 0;
  
                      try {
                          this.numChars = this.stream.read(this.buf);
                      } catch (IOException var2) {
                          throw new InputMismatchException();
                      }
  
                      if (this.numChars <= 0) {
                          return -1;
                      }
                  }
  
                  return this.buf[this.curChar++];
              }
          }
  
          public int nextInt() {
              int c;
              for (c = this.read(); isSpaceChar(c); c = this.read()) {
                  ;
              }
  
              byte sgn = 1;
              if (c == 45) {
                  sgn = -1;
                  c = this.read();
              }
  
              int res = 0;
  
              while (c >= 48 && c <= 57) {
                  res *= 10;
                  res += c - 48;
                  c = this.read();
                  if (isSpaceChar(c)) {
                      return res * sgn;
                  }
              }
  
              throw new InputMismatchException();
          }
  
          public static boolean isSpaceChar(int c) {
              return c == 32 || c == 10 || c == 13 || c == 9 || c == -1;
          }
  
      }
  
      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();
          }
  
          public void flush() {
              writer.flush();
          }
  
      }
  }
  
  