#include <cassert>
#include <iostream>
#include <set>
#include <vector>
#define ll long long
#define ln "\n"
#define ff first
#define ss second
#define ull unsigned ll
#define ld long double
const ll INF = 1e9;
const ll MOD = 1e9+7;
using namespace std;
void solve(){
ll n; cin >> n;
vector<ll> usd(n+1);
ll l=0, r=n;
vector<ll> moves;
while (l+1<r) {moves.push_back((r+l)/2); l=(r+l)/2;}
ll sp=1;
for (ll i=moves.size()-1; i>=0; i--){
if (sp+moves[i]<=n) sp+=moves[i];
else sp-=moves[i];
}
usd[sp]=1;
cout << "? " << sp << endl; ll _; cin >> _;
l=0, r=n; ll lp=sp;
while (l+1<r){
ll mid = (l+r)/2;
if (lp+mid<=n and !usd[lp+mid]){
lp=lp+mid;
cout << "? " << lp << endl;
}else{
if (lp-mid>=0 or !usd[lp-mid]){
lp-=mid;
}
cout << "? " << lp << endl;
}
usd[lp]=1;
ll x; cin >> x;
if (x) r=mid;
else l=mid;
}
cout << "= " << r << endl;
}
int main(){
ll t=1;
// cin >> t;
while (t--) solve();
}
