#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include <ext/rope>
using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
#define int long long
using i64 = long long;
mt19937_64 rd(time(0));
template <class K, class C = less<>> using paring_heap = __gnu_pbds::priority_queue<K, C>;
template <class K> using rb_tree = tree<K, null_type, less<>, rb_tree_tag, tree_order_statistics_node_update>;
template <class T, class ... A> void debug(const T & t, const A & ... a) { cerr << "[" << t, ((cerr << ", " << a), ...), cerr << "]\n"; }
const i64 mod = [](bool n) { return n ? 998244353 : 1e9 + 7; } (1);

void QAQ() {
    int n;
    cin >> n;

    int l = 1, r = n;

    auto query = [&](int x) {
        cout << "? " << x << endl;
        cin >> x;
        return x;
    };
    auto print = [&](int x) {
        cout << "! " << x << endl;
        return -1;
    };
    int lst = 0, now = 0;

    while (l < r) {
        int m = l + r >> 1;
        auto chk = [&](int m) {
            now = query(m);
            int tmp = now;
            if (now == abs(lst)) {
                return print(m);
            }
            now = query(r);
            if (now == tmp) {
                return print(r);
            } else if (now == lst) {
                r--;
                l = m + 1;
                lst = now;
            } else {
                lst = now;
                r = m - 1;
            }
            return 0;
        };
        if (chk(m) == -1) {
            return;
        }
    }

    print(l);
}

signed main() {
    cin.tie(0) -> sync_with_stdio(0);
    int t = 1;
    // cin >> t;
    for (cout << fixed << setprecision(12); t--; QAQ());
}