#include <bits/stdc++.h>
using namespace std;
long long que(long long va) {
  cout << "? " << va << endl;
  long long x;
  cin >> x;
  return x;
}
void print(long long x) {
  cout << "! " << x << endl;
  exit(0);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  cin >> n;
  if ((n / 2) % 2 == 1) {
    print(-1);
  }
  long long ha = n / 2;
  long long one = que(1), mid = que(1 + n / 2);
  long long inc = 1;
  if (mid > one)
    inc = 1;
  else
    inc = -1;
  if (one == mid) {
    print(1);
  }
  long long lo = 2, high = n / 2;
  while (lo < high) {
    long long m = (lo + high) / 2;
    long long valm = que(m);
    long long vnn = que(m + n / 2);
    if (valm == vnn) {
      print(m);
    }
    if (vnn > valm) {
      if (inc == 1) {
        lo = m + 1;
      } else
        high = m - 1;
    } else {
      if (inc == 1) {
        high = m - 1;
      } else
        lo = m + 1;
    }
  }
  print(lo);
}
