source_code: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.io.IOException;
  import java.io.BufferedReader;
  import java.io.Reader;
  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;
          MyInput in = new MyInput(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskB solver = new TaskB();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class TaskB {
          int n;
          MyInput in;
          PrintWriter out;
  
          public void solve(int testNumber, MyInput in, PrintWriter out) {
              this.in = in;
              this.out = out;
  
              n = in.nextInt();
  
              if (n / 2 % 2 == 1) {
                  answer(-1);
                  return;
              }
  
              int low = 0, high = n / 2;
              int diff = query(low + n / 2) - query(low);
              while (diff != 0) {
                  int mid = (low + high) / 2;
                  int d = query(mid + n / 2) - query(mid);
                  if (d == 0 || diff > 0 == d > 0) {
                      diff = d;
                      low = mid;
                  } else {
                      high = mid;
                  }
              }
              answer(low);
          }
  
          int query(int i) {
              out.println("? " + (i % n + 1));
              out.flush();
              return in.nextInt();
          }
  
          void answer(int i) {
              out.println("! " + (i < 0 ? i : (i % n + 1)));
          }
  
      }
  
      static class MyInput {
          private final BufferedReader in;
          private static int pos;
          private static int readLen;
          private static final char[] buffer = new char[1024 * 8];
          private static char[] str = new char[500 * 8 * 2];
          private static boolean[] isDigit = new boolean[256];
          private static boolean[] isSpace = new boolean[256];
          private static boolean[] isLineSep = new boolean[256];
  
          static {
              for (int i = 0; i < 10; i++) {
                  isDigit['0' + i] = true;
              }
              isDigit['-'] = true;
              isSpace[' '] = isSpace['\r'] = isSpace['\n'] = isSpace['\t'] = true;
              isLineSep['\r'] = isLineSep['\n'] = true;
          }
  
          public MyInput(InputStream is) {
              in = new BufferedReader(new InputStreamReader(is));
          }
  
          public int read() {
              if (pos >= readLen) {
                  pos = 0;
                  try {
                      readLen = in.read(buffer);
                  } catch (IOException e) {
                      throw new RuntimeException();
                  }
                  if (readLen <= 0) {
                      throw new MyInput.EndOfFileRuntimeException();
                  }
              }
              return buffer[pos++];
          }
  
          public int nextInt() {
              int len = 0;
              str[len++] = nextChar();
              len = reads(len, isSpace);
              int i = 0;
              int ret = 0;
              if (str[0] == '-') {
                  i = 1;
              }
              for (; i < len; i++) ret = ret * 10 + str[i] - '0';
              if (str[0] == '-') {
                  ret = -ret;
              }
              return ret;
          }
  
          public char nextChar() {
              while (true) {
                  final int c = read();
                  if (!isSpace[c]) {
                      return (char) c;
                  }
              }
          }
  
          int reads(int len, boolean[] accept) {
              try {
                  while (true) {
                      final int c = read();
                      if (accept[c]) {
                          break;
                      }
                      if (str.length == len) {
                          char[] rep = new char[str.length * 3 / 2];
                          System.arraycopy(str, 0, rep, 0, str.length);
                          str = rep;
                      }
                      str[len++] = (char) c;
                  }
              } catch (MyInput.EndOfFileRuntimeException e) {
              }
              return len;
          }
  
          static class EndOfFileRuntimeException extends RuntimeException {
          }
  
      }
  }
  
  