#include <bits/stdc++.h>
#pragma GCC optimize(O3)
using namespace std;
long long c = 0, d, e, a, b, res = 1e18, mas[200001], mas2[200000], f, z, res2,
          mas3[1000], array57[10][100000], mas4[100], r1, r2;
long long base = 57, p = 1e9 + 7, ans = 10000, sum, sum2, uk1, uk2, heh, heh2;
char ch;
string s, s2, s3, s4;
set<pair<long long, long long>> mset;
vector<pair<long long, long long>> vec;
deque<long long> ord, answer[5000], vec2;
map<pair<long long, long long>, long long> map57;
multiset<long long> mset57;
long long map1[5000], map2[5000][101];
long long get_sum(vector<long long> a) {
  long long answer2 = 0, r = 1;
  for (int i = 0; i < a.size(); i++) {
    answer2 += r * a[i];
    r *= 2;
  }
  return answer2;
}
signed main() {
  srand(time(0));
  cin >> a;
  if (a % 4 == 2) {
    cout << "! -1" << endl;
    return 0;
  }
  cout << "? " << 1 << endl;
  cin >> b;
  cout << "? " << 1 + a / 2 << endl;
  cin >> c;
  d = (b - c) / 2;
  if (d == 0) {
    cout << "! 1" << endl;
    return 0;
  }
  if (d > 0) {
    long long uk1 = 1, uk2 = 1 + a / 2;
    while (true) {
      res = (uk1 + uk2) / 2;
      cout << "? " << res << endl;
      cin >> b;
      cout << "? " << res + a / 2 << endl;
      cin >> c;
      d = (b - c) / 2;
      if (d > 0) {
        uk1 = res;
      }
      if (d < 0) {
        uk2 = res;
      }
      if (d == 0) {
        cout << "! " << res << endl;
        return 0;
      }
    }
  } else {
    long long uk1 = 1, uk2 = 1 + a / 2;
    while (true) {
      res = (uk1 + uk2) / 2;
      cout << "? " << res << endl;
      cin >> b;
      cout << "? " << res + a / 2 << endl;
      cin >> c;
      d = (b - c) / 2;
      if (d > 0) {
        uk2 = res;
      }
      if (d < 0) {
        uk1 = res;
      }
      if (d == 0) {
        cout << "! " << res << endl;
        return 0;
      }
    }
  }
  cin >> a;
  return 0;
}
