#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  cout << "? 1\n";
  fflush(stdout);
  int a, b;
  cin >> a;
  cout << "? " << 1 + n / 2 << "\n";
  cin >> b;
  fflush(stdout);
  if ((abs(a - b)) % 2 == 1) {
    cout << "! -1\n";
    fflush(stdout);
    return 0;
  }
  if (a == b) {
    cout << "! 1\n";
    fflush(stdout);
    return 0;
  }
  int l = 1, r = n / 2;
  int pr = a - b;
  while (1) {
    int m = (l + r) / 2;
    cout << "? " << m << "\n";
    fflush(stdout);
    cin >> a;
    cout << "? " << m + n / 2 << "\n";
    fflush(stdout);
    cin >> b;
    if (a == b) {
      cout << "! " << m << "\n";
      fflush(stdout);
      return 0;
    }
    if (pr < 0 && a - b < 0) {
      l = m + 1;
    } else {
      if (pr > 0 && a - b > 0) {
        l = m + 1;
      } else {
        r = m - 1;
      }
    }
  }
}
