#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    int res = -1;
    if (n%2) res = n-1;
    for (int i = 0; i+1 < n; i += 2) {
        cout << "? " << i << " " << i+1 << endl;
        int x; cin >> x;
        if (x == i) {
            res = i+1;
        }
        else if (x == i+1) {
            res = i;
        }
    }
    cout << "! " << res << endl;
}