source_code: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  import java.io.IOException;
  import java.io.BufferedReader;
  import java.io.InputStreamReader;
  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;
          InputReader in = new InputReader(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskB solver = new TaskB();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class TaskB {
          int N;
  
          public void solve(int testNumber, InputReader in, PrintWriter out) {
              N = in.nextInt();
              int low = 1;
              int lowVal = getval(1, out, in);
              int high = N / 2 + 1;
              int highVal = -lowVal;
              if (Math.abs(lowVal) % 2 == 1) {
                  out.println("! -1");
                  out.flush();
              } else {
                  while (low < high) {
                      int mid = (low + high) / 2;
                      int a = getval(mid, out, in);
                      if (Integer.signum(a) == 0) {
                          out.println("! " + mid);
                          out.flush();
                          return;
                      } else {
                          if (Integer.signum(a) == Integer.signum(lowVal)) {
                              low = mid + 1;
                          } else {
                              high = mid;
                          }
                      }
                  }
                  out.println("! " + low);
                  out.flush();
              }
          }
  
          int getval(int i, PrintWriter out, InputReader in) {
              out.println("? " + i);
              out.flush();
              int a = in.nextInt();
              out.println("? " + (i + N / 2));
              out.flush();
              int b = in.nextInt();
              return a - b;
          }
  
      }
  
      static class InputReader {
          public BufferedReader reader;
          public StringTokenizer tokenizer;
  
          public InputReader(InputStream stream) {
              reader = new BufferedReader(new InputStreamReader(stream), 32768);
              tokenizer = null;
          }
  
          public String next() {
              while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                  try {
                      tokenizer = new StringTokenizer(reader.readLine());
                  } catch (IOException e) {
                      throw new RuntimeException(e);
                  }
              }
              return tokenizer.nextToken();
          }
  
          public int nextInt() {
              return Integer.parseInt(next());
          }
  
      }
  }
  
  