#include <bits/stdc++.h>
using namespace std;
int n;
long long get_b(int i) {
  long long t, y;
  cout << "? " << i << '\n';
  cin >> t;
  cout << "? " << i + n / 2 << '\n';
  cin >> y;
  return t - y;
}
int main() {
  cin >> n;
  int l = 1, r = 1 + (n / 2);
  long long x = get_b(1);
  long long y = -x;
  if (x % 2 != 0) {
    cout << "! " << -1;
    return 0;
  }
  while (true) {
    if (x == 0) {
      cout << "! " << l << '\n';
      return 0;
    } else if (y == 0) {
      cout << "! " << r << '\n';
      return 0;
    }
    int m = (l + r) >> 1;
    long long temp = get_b(m);
    if (temp < 0) {
      if (x < 0) {
        l = m;
        x = temp;
      } else {
        r = m;
        y = temp;
      }
    } else {
      if (x > 0) {
        l = m;
        x = temp;
      } else {
        r = m;
        y = temp;
      }
    }
  }
}
