#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for (long long i=0;i<(long long)(n);i++)
#define all(v) v.begin(),v.end()
using ll=long long;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll,ll>;
const ll INF=(1ll<<60);
template<class T> void chmin(T &a,T b){
    if(a>b){
        a=b;
    }
}
template<class T> void chmax(T &a,T b){
    if(a<b){
        a=b;
    }
}
int main(){
    ll n;
    cin >> n;
    ll k=n/2;
    ll now=0;
    while(k--){
        cout << "? " << now << " " << now+1 << endl;
        ll z;
        cin >> z;
        if(z==now){
            cout << "! " << now+1 << endl;
            return 0;
        }else if(z==now+1){
            cout << "! " << now << endl;
            return 0;
        }
        now+=2;
    }
    cout << "! " << n-1 << endl;
}