#include<bits/stdc++.h>
using namespace std;
using ll =long long;
ll ask(ll d)
{
    cout<<"? "<<d<<endl;
    ll n;
    cin>>n;
    return n;
}
void solve()
{
    ll n;
    cin>>n;
    ll l=1,r=n;
    ll ls=0,rs=0;
    while(l<=r)
    {
        ll mid=(l+r)>>1;
        ll d=ask(mid);
        if(d==ls-rs)
        {
            cout<<"! "<<mid<<endl;
            return;
        }else  if(d>ls-rs)
        {
            ls++;
            l=mid+1;
        }else{
            rs++;
            r=mid-1;
        }
    }
}
int main(){
    ll t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
    return 0;
}