#include <bits/stdc++.h>
using namespace std;
int ask(int x) {
  cout << "? " << x << endl;
  fflush(stdout);
  cin >> x;
  return x;
}
void print(int x) {
  cout << "! " << x << endl;
  exit(0);
}
int main() {
  int n, hf;
  cin >> n, hf = n >> 1;
  if (hf & 1) print(-1);
  int a = ask(1), b = ask(1 + hf);
  if (a == b) print(1);
  int l, r;
  if (a < b)
    l = 2, r = hf;
  else
    l = hf + 2, r = n;
  while (l < r) {
    int mid = (l + r) >> 1, pim = (mid - 1 + hf + n) % n + 1;
    a = ask(mid), b = ask(pim);
    if (a == b) print(mid);
    if (a < b)
      l = mid + 1;
    else
      r = mid - 1;
  }
  print(l);
  return 0;
}
