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
   *
   * @author kessido
   */
  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);
          BTheHat solver = new BTheHat();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class BTheHat {
          PrintWriter out;
          InputReader in;
          int n;
  
          public void solve(int testNumber, InputReader in, PrintWriter out) {
              this.out = out;
              this.in = in;
              n = in.NextInt();
              int desiredPair = -1;
              int result = query(1);
              if (result != 0) {
                  int l = 2, r = 1 + n / 2;
                  while (l < r) {
                      int m = (l + r) / 2;
                      int mRes = query(m);
                      if (mRes == 0) {
                          desiredPair = m;
                          break;
                      } else if (mRes == result) {
                          l = m + 1;
                      } else {
                          r = m;
                      }
                  }
              } else {
                  desiredPair = 1;
              }
              out.println("! " + desiredPair);
          }
  
          private int query(int i) {
              int iV = queryValue(i);
              int iN2V = queryValue(i + n / 2);
              if (iV < iN2V) {
                  return -1;
              } else if (iV > iN2V) {
                  return 1;
              }
              return 0;
          }
  
          private int queryValue(int i) {
              out.println("? " + i);
              out.flush();
              return in.NextInt();
          }
  
      }
  
      static class InputReader {
          BufferedReader reader;
          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(), " \t\n\r\f,");
                  } catch (IOException e) {
                      throw new RuntimeException(e);
                  }
              }
              return tokenizer.nextToken();
          }
  
          public int NextInt() {
              return Integer.parseInt(next());
          }
  
      }
  }
  
  