#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  scanf("%d", &n);
  if (n % 4 == 2) {
    cout << "! -1\n";
    cout.flush();
    return 0;
  }
  int l = 1, r = n / 2 + 1;
  int l1, r1;
  cout << "? " << l << "\n";
  cout.flush();
  scanf("%d", &l1);
  cout << "? " << r << "\n";
  cout.flush();
  scanf("%d", &r1);
  l1 = l1 - r1;
  r1 = -l1;
  if (l1 == 0) {
    cout << "! " << l << "\n";
    cout.flush();
    return 0;
  }
  while (l + 2 < r) {
    int mid = (l + r) >> 1;
    int m1, m2;
    cout << "? " << mid << "\n";
    cout.flush();
    scanf("%d", &m1);
    cout << "? " << mid + n / 2 << "\n";
    cout.flush();
    scanf("%d", &m2);
    m1 -= m2;
    if (m1 == 0) {
      cout << "! " << mid << "\n";
      cout.flush();
      return 0;
    }
    if (m1 * l1 < 0) {
      r = mid;
      r1 = m1;
    } else {
      l = mid;
      l1 = m1;
    }
  }
  for (; l <= r; ++l) {
    cout << "? " << l << "\n";
    cout.flush();
    scanf("%d", &l1);
    cout << "? " << l + n / 2 << "\n";
    cout.flush();
    ;
    scanf("%d", &r1);
    ;
    if (l1 == r1) {
      cout << "! " << l << "\n";
      cout.flush();
      ;
      return 0;
    }
  }
  return 0;
}
