#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using ui = unsigned int;
using ull = unsigned long long;
using pui = pair<ui,ui>;
using pull = pair<ull,ull>;
template<typename T>
using vc = std::vector<T>;
#define YES cout << "YES\n"
#define NO cout << "NO\n"
#define yes cout << "yes\n"
#define no cout << "no\n"
#define Yes cout << "Yes\n"
#define No cout << "No\n"
template<typename T> void sc(T &x) { cin >> x; }
template<typename T, typename U> void sc(pair<T,U> &p) { sc(p.first), sc(p.second); }
template<typename T> void sc(vector<T> &v) { for (auto &x : v) sc(x); }
template<typename... A> void sc(A&... a) { (sc(a), ...); }
template<typename T> void _pt(const T &x){cout << x;}
template<typename T, typename U> void _pt(const pair<T,U> &p){_pt(p.first); cout << ' '; _pt(p.second);}
template<typename T> void _pt(const vector<T> &v){bool f=1; for(auto &x:v){if(!f) cout<<' '; _pt(x); f=0;}}
template<typename... A> void pt(const A&... a){(_pt(a), ...);}
template<typename T, typename U> bool umin(T &a, const U &b) { return b < a ? (a = b, true) : false; }
template<typename T, typename U> bool umax(T &a, const U &b) { return b > a ? (a = b, true) : false; }
#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define FOR1(n) for (int i=0; i<(n); ++i)
#define FOR2(i,n) for (int i=0; i<(n); ++i)
#define FOR3(i,l,r) for (int i=(l); i<(r); ++i)
#define FOR4(i,l,r,k) for (int i=(l); i<(r); i+=(k))
#define FOR(...) GET_MACRO(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define ROF1(n) for (int i=(n)-1; i>=0; --i)
#define ROF2(i,n) for (int i=(n)-1; i>=0; --i)
#define ROF3(i,l,r) for (int i=(r)-1; i>=(l); --i)
#define ROF4(i,l,r,k) for (int i=(r)-1; i>=(l); i-=(k))
#define ROF(...) GET_MACRO(__VA_ARGS__, ROF4, ROF3, ROF2, ROF1)(__VA_ARGS__)
#define all(c) (c).begin(), (c).end()
#define allr(c) (c).rbegin(), (c).rend()
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
#define pb push_back
#define trav(x,a) for (auto &x : (a))
#define sz(a) ((int)(a).size())
#define mem(a,v) memset(a, v, sizeof(a))
#define nl pt("\n")
ll n;
void solve() {
ll l = 1, r = n - 1;
ll p = n, t = 0, d = 0;
auto ask = [&](ll x) {
pt("? ", x); nl; cout << flush;
sc(x);
return x;
};
while(l <= r) {
ll m = (l + r) >> 1;
d += t ? -m : m;
umin(p, n - d);
t ^= 1;
l = m + 1;
}
l = 1; t = 0;
ll ans = n;
ask(p);
while(l <= r) {
ll m = (l + r) >> 1;
ll np = p + (t ? -m : m);
if(ask(np)) ans = m, r = m - 1;
else l = m + 1;
t ^= 1; p = np;
}
pt("= ", ans); nl; cout << flush;
}
int32_t main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
sc(n);
solve();
return 0;
}
