#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
const long long maxn = (long long)3100;
const double EPS = 1e-9;
const long long INF = 1e18 + 18;
const long long MOD = (long long)1e9 + 7;
long long n;
long long ask(long long i) {
  long long inp;
  cout << "? " << i + 1 << endl;
  fflush(stdout);
  cin >> inp;
  return inp;
}
signed main() {
  cin >> n;
  long long l = 0, r = n - 1;
  while (r - l > 1) {
    long long m = (r + l) >> 1;
    long long lquery = ask(m), rquery = ask((m + n / 2) % n);
    if (lquery < rquery) {
      r = m;
    } else if (lquery == rquery) {
      cout << "! " << m + 1 << endl;
      fflush(stdout);
      return 0;
    } else
      l = m;
  }
  cout << "! -1" << endl;
  fflush(stdout);
  return 0;
}
