source_code: import java.util.*;
  import java.io.*;
  import java.math.*;
  public class Main
  {
      static class Reader 
      { 
          private InputStream mIs;private byte[] buf = new byte[1024];private int curChar,numChars;public Reader() { this(System.in); }public Reader(InputStream is) { mIs = is;} 
          public int read() {if (numChars == -1) throw new InputMismatchException();if (curChar >= numChars) {curChar = 0;try { numChars = mIs.read(buf);} catch (IOException e) { throw new InputMismatchException();}if (numChars <= 0) return -1; }return buf[curChar++];} 
          public String nextLine(){int c = read();while (isSpaceChar(c)) c = read();StringBuilder res = new StringBuilder();do {res.appendCodePoint(c);c = read();}while (!isEndOfLine(c));return res.toString() ;} 
          public String s(){int c = read();while (isSpaceChar(c)) c = read();StringBuilder res = new StringBuilder();do {res.appendCodePoint(c);c = read();}while (!isSpaceChar(c));return res.toString();} 
          public long l(){int c = read();while (isSpaceChar(c)) c = read();int sgn = 1;if (c == '-') { sgn = -1 ; c = read() ; }long res = 0; do{ if (c < '0' || c > '9') throw new InputMismatchException();res *= 10 ; res += c - '0' ; c = read();}while(!isSpaceChar(c));return res * sgn;} 
          public int i(){int c = read() ;while (isSpaceChar(c)) c = read();int sgn = 1;if (c == '-') { sgn = -1 ; c = read() ; }int res = 0;do{if (c < '0' || c > '9') throw new InputMismatchException();res *= 10 ; res += c - '0' ; c = read() ;}while(!isSpaceChar(c));return res * sgn;} 
          public double d() throws IOException {return Double.parseDouble(s()) ;}
          public boolean isSpaceChar(int c) { return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1; } 
          public boolean isEndOfLine(int c) { return c == '\n' || c == '\r' || c == -1; } 
          public int[] arr(int n){int[] ret = new int[n];for (int i = 0; i < n; i++) {ret[i] = i();}return ret;}
      }
      
      
   
             //       |----|       /\      |    |   -----   |
             //       |   /       /  \     |    |     |     |
             //       |--/       /----\    |----|     |     |
             //       |   \     /      \   |    |     |     |
             //       |    \   /        \  |    |   -----   -------
  
      public static void main(String[] args)throws IOException
      {
          PrintWriter out= new PrintWriter(System.out);
          Reader sc=new Reader();
          int n=sc.i();
          System.out.println("? "+1);
          int a=sc.i();
          System.out.println("? "+(1+n/2));
          int b=sc.i();
          if(a==b)
          {
              System.out.println("! "+1);
              System.exit(0);
          }
          int inv=0;
          if(a>b)
          inv=1;
          
          int low=2;
          int high=n/2;
          int q=0;
          while(low<=high)
          {
              if(q==60)
              break;
              int mid=(low+high)/2;
              System.out.println("? "+mid);
              a=sc.i();
              System.out.println("? "+(mid+n/2));
              b=sc.i();
              if(a==b)
              {
                  System.out.println("! "+mid);
                  System.exit(0);
              }
              else if(a<b)
              {
                  if(inv==0)
                  low=mid+1;
                  else
                  high=mid-1;
              }
              
              else
              {
                  if(inv==0)
                  high=mid-1;
                  else
                  low=mid+1;
              }
              q++;
          }
          System.out.println("! -1");
          out.flush();
      }
  }