#include <bits/stdc++.h>
using namespace std;
int n;
void p(int k) { cout << "? " << k + 1 << endl; }
void p2(int k) { cout << "! " << k + 1 << endl; }
int gg(int k) {
  int ch1, ch2;
  p(k);
  cin >> ch1;
  p(k + n / 2);
  cin >> ch2;
  return ch1 - ch2;
}
signed main() {
  cin >> n;
  if (n % 4 != 0) {
    cout << "! -1" << endl;
    return 0;
  }
  int l = 0, r = l + n / 2, chl, chr, raznl = 0, raznr = 0, ch, razn = -1, pos;
  raznl = gg(l);
  raznr = -raznl;
  if (raznl == 0) {
    p2(l);
    return 0;
  }
  while (razn != 0) {
    pos = (l + r) / 2;
    razn = gg(pos);
    if (razn == 0) {
      p2(pos);
      return 0;
    }
    if (abs(razn) / razn == abs(raznl) / raznl) {
      l = pos;
      raznl = razn;
    } else {
      r = pos;
      raznr = razn;
    }
  }
  return 0;
}
