#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pci = pair<char, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vpii = vector<pii>;
const ll infll = 1e18 + 3;
const int maxn = 1e6 + 77;
const int nmax = 1e6 + 77;
const ll basell = 1e18 + 3;
const int mod = 1e9 + 7;
const ld eps = 1e-7;
const int inf = 1009000999;
const int nv = 100100;
const int baseint = 1000200013;
const ld PI = acos(-1.0);
inline bool EQ(ld a, ld b) { return fabs(a - b) < 1e-9; }
int n;
int ask(int x) {
  cout << "? " << (x % n) + 1 << '\n';
  int a;
  cin >> a;
  return a;
}
void ans(int x) {
  cout << "! " << (x % n) + 1 << '\n';
  exit(0);
}
int sgn(int x) {
  if (x == 0) {
    return 0;
  }
  return x < 0 ? -1 : 1;
}
int inline solve() {
  cin >> n;
  int m = n / 2;
  int s = ask(0);
  int t = ask(m);
  if (s == t) ans(0);
  if ((s - t) % 2 != 0) {
    return cout << "! -1\n", 0;
  }
  int l = 0, r = m;
  while (r - l > 1) {
    int mid = (r + l) / 2;
    int a = ask(mid);
    int b = ask(mid + m);
    if (a == b) ans(mid);
    if (sgn(a - b) != sgn(s - t)) {
      r = mid;
    } else {
      l = mid;
    }
  }
  if (ask(l) == ask(l + m)) {
    ans(l);
  } else {
    ans(r);
  }
}
int32_t main() {
  solve();
  return 0;
}
