#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  if (n % 4) {
    cout << "! -1\n";
    return 0;
  }
  int l = 1, r = 1 + n / 2;
  function<int(int i)> ask = [&](int i) {
    int p = (i + n / 2);
    if (p > n) p -= n;
    int a, b;
    cout << "? " << i << "\n"
         << "? " << p << "\n";
    cout.flush();
    cin >> a >> b;
    return a - b;
  };
  int lv = ask(l), rv = -lv;
  if (lv == 0) {
    cout << "! " << l << endl;
    return 0;
  }
  while (r - l > 1) {
    int mid = (l + r) / 2;
    int midV = ask(mid);
    if (midV == 0) {
      cout << "! " << mid << endl;
      return 0;
    } else if (midV / abs(midV) == lv / abs(lv)) {
      l = mid;
      lv = midV;
    } else
      r = mid, rv = midV;
  }
  lv = ask(l), rv = ask(r);
  if (lv == 0) {
    cout << "! " << l << endl;
    return 0;
  }
  if (rv == 0) {
    cout << "! " << r << endl;
    return 0;
  }
}
