#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1ULL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll gcd(ll a, ll b){return __gcd(abs(a), abs(b));}
ll lcm(ll a, ll b){return abs(a) / gcd(a, b) * abs(b);}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ull mask){return __builtin_popcountll(mask);}
int ctz(ull mask){return __builtin_ctzll(mask);}
int logOf(ull mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
int solve(ll n);
// namespace InteractorLib{
// set<ll> S;
// ll last = 0;
// ll c;
// bool valid;
// int query(ll x){
// if (S.insert(x).second == false){
// valid = false;
// }
// ll diff = abs(last - x);
// last = x;
// return diff >= c;
// }
// int grade(){
// ll n = rngesus(1, 125);
// c = rngesus(1, n);
// S.clear(); last = 0;
// valid = true;
// ll ans = solve(n);
// if (valid == false) return -1;
// if (ans != c) return -1;
// return S.size();
// }
// };
// using namespace InteractorLib;
int query(ll x){
cout << "? " << x << endl;
int verdict; cin >> verdict;
return verdict;
}
void update(pair<ll, ll> &p, ll x, bool verdict){
if (verdict) minimize(p.second, x);
else maximize(p.first, x + 1);
}
int solve(ll n){
pair<ll, ll> ans = make_pair(1, n);
ll last = -1;
if (n <= 64){
ll l = 1, r = n;
while(l <= r){
int verdict = query(l);
if (last != -1) update(ans, abs(last - l), verdict);
last = l++;
if (l <= r){
verdict = query(r);
update(ans, abs(last - r), verdict);
last = r--;
}
}
}
else{
ll l = 32, r = n - 32 + 1;
query(l);
int verdict = query(r);
update(ans, abs(r - l), verdict);
last = r;
if (verdict){
l++; r--;
while(l <= r){
int verdict = query(l);
update(ans, abs(last - l), verdict);
last = l++;
if (l <= r){
verdict = query(r);
update(ans, abs(last - r), verdict);
last = r--;
}
}
}
else{
l--; r++;
while(l > 0){
int verdict = query(l);
update(ans, abs(last - l), verdict);
last = l--;
verdict = query(r);
update(ans, abs(last - r), verdict);
last = r++;
}
}
}
return ans.first;
}
int main(void){
ios::sync_with_stdio(0);cin.tie(0); cout.tie(0);
clock_t start = clock();
// for(int t = 0; t <= 1000; ++t) cout << grade() << "\n";
ll n; cin >> n;
int ans = solve(n);
cout << "= " << ans << endl;
cerr << "Time elapsed: " << clock() - start << "ms!\n";
return 0;
}
