#include <bitset>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#include <immintrin.h>
using namespace std;
typedef long long ll;
#define int ll
const int inf = int(1e18);
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> p2;
#define rep(i, high) for (int i = 0; i < (high); i++)
#define repp(i, low, high) for (int i = (low); i < (high); i++)
#define repe(i, container) for (auto& i : container)
#define sz(container) ((int)container.size())
#define all(x) begin(x),end(x)
inline void fast() { cin.tie(0)->sync_with_stdio(0); }
#if _LOCAL
#define assert(x) if (!(x)) __debugbreak()
#endif
int n;
int numguess = 0;
#if !_LOCAL
int ask(int x)
{
cout << "? " << x << endl;
int resp;
cin >> resp;
return resp;
}
#else
int secret=-1;
void init()
{
mt19937 rng(time(0));
secret = n;//uniform_int_distribution<int>(1, n)(rng);
cout << "SECRET=" << secret << "\n";
}
int last = -1000;
int ask(int x)
{
numguess++;
//cout << "ASK " << x << ",DIFF=" << abs(x - last) << "\n";
if (secret == -1) init();
int ret;
if (last == -1000) ret = 0;
else
{
ret = abs(x - last) >= secret;
}
last = x;
return ret;
}
#endif
signed main()
{
fast();
/*vi nums;
int lo = 1;
int hi = 239578;
while (lo+1<hi)
{
int mid = (lo + hi) / 2;
nums.push_back(mid);
lo = mid;
}
repe(v, nums) cout << v << "\n";
int ans = 0;
rep(i, sz(nums)) ans += (i % 2 ? 1 : -1) * nums[i];
cout << ans << " " << abs(ans-hi/3);
cout << "";
return 0;*/
//n = int(1e9);
////cin >> n;
//if (n<=64)
//{
// rep(i, n / 2)
// {
// if (ask(i + 1) == 0 && i)
// {
// cout << "= " << abs(n-(i-1)-i+1)-1 << endl;
// return 0;
// }
// if (ask(n - i) == 0)
// {
// cout << "= " << abs(n - i - (i + 1)) + 1 << endl;
// return 0;
// }
// }
//}
cin >> n;
ask(n/3);
int last = n/3;
int lo = 1;
int hi = n;
int ind = 0;
while (lo+1<hi)
{
int mid = (lo + hi) / 2;
int g;
if (ind % 2 == 0) g = last + mid;
else g = last-mid;
if (ask(g))
{
hi = mid;
}
else lo = mid;
last = g;
ind++;
}
cout << "= " << hi << endl;
cerr << "G= " << numguess;
return 0;
}
