#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }


int Ask(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  int ret;
  scanf("%d", &ret);
  return ret;
}
void Answer(int x) {
  printf("! %d\n", x);
  fflush(stdout);
  exit(0);
}

int main() {
  int N;
  scanf("%d", &N);
  
  int last = 0;
  
  vector<int> cands;
  auto brute = [&]() -> void {
    const int len = cands.size();
    for (int i = 0; i < len - 1; ++i) {
      const int res = Ask(cands[i]);
      if (last == res) Answer(cands[i]);
      last = res;
    }
    Answer(cands[len - 1]);
  };
  
  int L = 1, R = N;
  int A, B;
  for (; ; ) {
    const int l = (L + L + L + R) / 4;
    const int r = (L + R + R + R) / 4;
    if (l == r) {
      for (int x = L; x <= R; ++x) cands.push_back(x);
      brute();
    }
    const int f = Ask(l); if (last == f) { Answer(l); } last = f;
    const int g = Ask(r); if (last == g) { Answer(r); } last = g;
    if (g) {
      A = l - 1;
      B = r + 1;
      break;
    } else {
      L = l + 1;
      R = r - 1;
    }
  }
// cerr<<"waf "<<L<<" "<<A<<" "<<B<<" "<<R<<endl;
  
  for (; L <= A && B <= R; ) {
    const int l = (L + A) / 2;
    const int r = (B + R) / 2;
    const int e = last;
    const int f = Ask(l); if (last == f) { Answer(l); } last = f;
    const int g = Ask(r); if (last == g) { Answer(r); } last = g;
    if (e < g) {
      A = l - 1;
      B = r + 1;
    } else {
      L = l + 1;
      R = r - 1;
    }
  }
  
  for (int x = L; x <= A; ++x) cands.push_back(x);
  for (int x = B; x <= R; ++x) cands.push_back(x);
  brute();
  
  return 0;
}