#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define fi first
#define se second
#define vi vector<int>
#define vll vector<long long>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::high_resolution_clock::now().time_since_epoch().count());
inline int rand(int l,int r){return uniform_int_distribution<int>(l, r)(rng);}
inline ll rand(ll l,ll r){return uniform_int_distribution<ll>(l, r)(rng64);}
#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif
struct custom_hash {
static uint64_t splitmix64(uint64_t x) {
x += 0x9e3779b97f4a7c15;
x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
return x ^ (x >> 31);
}
size_t operator()(uint64_t x) const {
static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
return splitmix64(x + FIXED_RANDOM);
}
};
struct pair_hash{
size_t operator()(const pair<int,int>&x)const{
return hash<long long>()(((long long)x.first)^(((long long)x.second)<<32));
}
};
int ask(ll k){
cout << "? " << k << endl;
cout.flush();
int res;
cin >> res;
return res;
}
int main(){
/*ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);*/
ll n;
cin >> n;
vll vec;
ll x1 = 0LL;
ll x2 = n;
while(x2 > x1 + 1){
vec.pb((x1 + x2) / 2);
x1 = (x1 + x2) / 2;
}
reverse(all(vec));
ll p = 1LL;
for(auto ele : vec){
if(ele + p <= n){
p += ele;
}else{
p -= ele;
}
}
set<ll> used;
ask(p);
used.insert(p);
ll lo = 0LL;
ll hi = n;
ll mid;
while(lo + 1< hi){
mid = (lo + hi) / 2;
if(p + mid <= n && used.find(p + mid) == used.end()){
p = (ll)p + mid;
}else{
p = (ll)p - mid;
}
if(ask(p)){
hi = mid;
}else{
lo = mid;
}
used.insert(p);
}
cout << "= " << hi << endl;
cout.flush();
return 0;
}
