#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
bool ask(ll x){
cout << "? " << x << endl;
int k;
cin >> k;
return k == 1;
}
ll n;
namespace sub1{
void solve(){
ask(1);
for(int i = n, cur = 1; i > 1; i--){
if((n ^ i ^ 1) & 1){
if(!ask(cur += i - 1)){
return void(cout << "= " << i);
}
}
else if(!ask(cur -= i - 1)){
return void(cout << "= " << i);
}
}
cout << "= 1";
}
}
namespace sub2345{
void solve(){
vector<ll>a;
ll low = 1, high = n - 1, ans = n;
while(low <= high){
ll mid = (low + high) >> 1LL;
a.push_back(mid);
low = mid + 1;
}
reverse(a.begin(), a.end());
int sign = 1;
ll p = 1;
for(ll& x : a){
p += x * sign;
sign = -sign;
}
ask(p);
for(low = 1, high = n - 1; low <= high; sign = -sign){
ll mid = (low + high) >> 1LL;
if(ask(p += mid * sign)){
high = (ans = mid) - 1;
}
else{
low = mid + 1;
}
}
cout << "= " << ans;
}
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
if(n <= 64){
sub1::solve();
}
else{
sub2345::solve();
}
}
