#ifndef LOCAL
#pragma GCC Optimize("O3,Ofast,unroll-loops")
#pragma GCC Target("bmi,bmi2,avx,avx2")
#endif
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define int ll
#define f first
#define s second
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin() (x).rend()
mt19937 rnd(11);
int n, C;
int lst;
set<int> was;
int ask(int x) {
assert(was.count(x) == 0);
assert(x >= 0);
assert(x < n);
was.insert(x);
cout << "? " << x + 1 << endl;
int ans;
#ifdef LOCAL
ans = (abs(x - lst) >= C);
lst = x;
#else
cin >> ans;
#endif
return ans;
}
void solve() {
was.clear();
cin >> n;
int x = 0;
int left = 0, right = 0;
{
int now = 0, d = 1;
int l = 0, r = n;
auto check = [&](int len) {
now += d * len;
d *= -1;
if (now > 0) {
right = max(right, now);
} else {
left = max(left, -now);
}
return false;
};
while (r - l > 1) {
int m = (l + r) / 2;
if (check(m)) {
r = m;
} else {
l = m;
}
}
assert(right + left <= n);
x = left;
}
int now = x, d = 1;
ask(now);
int l = 0, r = n;
auto check = [&](int len) {
now += d * len;
d *= -1;
return ask(now);
};
while (r - l > 1) {
int m = (l + r) / 2;
if (check(m)) {
r = m;
} else {
l = m;
}
}
cout << "= " << r << endl;
}
signed main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#else
ios::sync_with_stdio(false);
cin.tie(0);
#endif
int t = 1;
// cin >> t;
while (t--) {
solve();
}
}
