#include <bits/stdc++.h>
using namespace std;
const int mod = 998244353;
inline int read() {
    int x = 0, f = 1;
    char c = getchar();
    while (c < '0' || c > '9')
        f = (c == '-') ? -1 : 1, c = getchar();
    while (c >= '0' && c <= '9')
        x = x * 10 + c - 48, c = getchar();
    return f * x;
}
signed main() {
    int n, l = 2, r, ans = 0, x, last = 0;
    cin >> n, r = n;
    cout << "? 1" << endl;
    cin >> last;
    if (last == 0) {
        cout << "! 1" << endl;
        return 0;
    }
    while (l <= r) {
        int mid = (l + r) >> 1;
        cout << "? " << mid << endl;
        cin >> x;
        if (x == last) {
            cout << "! " << mid << endl;
            return 0;
        }
        if (x == last - 1) {
            last = x;
            cout << "? " << l + 1 << endl;
            cin >> x;
            if (x == last) {
                cout << "! " << l + 1 << endl;
                return 0;
            }
            r = mid - 1;
            l++;
        } else {
            last = x;
            cout << "? " << mid + 1 << endl;
            cin >> x;
            if (x == last) {
                cout << "! " << mid + 1 << endl;
                return 0;
            }
            l = mid + 2;
        }
        last = x;
    }
}