source_code: import java.io.*;
  import java.util.*;
  
  public class B {
  	public static void main(String[] args) {
  		InputStream inputStream = System.in;
  		OutputStream outputStream = System.out;
  		InputReader in = new InputReader(inputStream);
  		PrintWriter out = new PrintWriter(outputStream);
  		Task solver = new Task();
  		solver.solve(1, in, out);
  		out.close();
  	}
  
  	static class Task {
  		int n = 0;
  		int query(int p, InputReader in) {
  			p %= n;
  			if (p <= 0) p += n;
  			System.out.println("? " + p);
  			System.out.flush();
  			int x = in.nextInt();
  			return x;
  		}
  		public void solve(int testNumber, InputReader in, PrintWriter out) {
  			n = in.nextInt();
  			if (n % 4 != 0) {
  				out.println("! -1");
  				return;
  			}
  			int p = query(0, in);
  			int q = query(n / 2, in);
  			int l = 0;
  			int r = n / 2;
  			if (p == q) {
  				out.println("! " + (n / 2));
  				return;
  			}
  			while (l + 1 < r) {
  				int mid = (l + r) / 2;
  				int u = query(mid, in);
  				int v = query(mid + n / 2, in);
  				if (u == v) {
  					out.println("! " + (mid + n / 2));
  					return;
  				}
  				if ((p < q) == (u < v)) {
  					l = mid;
  				} else {
  					r = mid;
  				}
  			}
  		}
  	}
  
  	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());
  		}
  
  		public long nextLong() {
  			return Long.parseLong(next());
  		}
  
  	}
  }