#include <bits/stdc++.h>
using namespace std;
int get(int x) {
  x++;
  cout << "? " << x << endl;
  int ret;
  cin >> ret;
  return ret;
}
void ans(int x) {
  x++;
  cout << "! " << x << endl;
}
int main() {
  int n;
  cin >> n;
  int lo = 0;
  int hi = n / 2;
  int dif = get(lo) - get(hi);
  if (dif == 0) {
    ans(0);
    return 0;
  }
  bool ok = (((dif % 2) + 2) % 2) == 0;
  if (!ok) {
    ans(-2);
    return 0;
  }
  bool lpos = dif > 0;
  while (lo < hi) {
    int mid = (lo + hi) / 2;
    int dif2 = get(mid) - get((mid + n / 2) % n);
    if (dif2 == 0) {
      ans(mid);
      return 0;
    }
    if (dif2 > 0 == lpos) {
      lo = mid + 1;
    } else {
      hi = mid - 1;
    }
  }
  ans(lo);
  return 0;
}
