#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int ask (ll x) {
cout << "? " << x << endl;
int res;
cin >> res;
return res;
}
int32_t main () {
// ios_base::sync_with_stdio(0);
// cin.tie(0);
// cout.tie(0);
ll N;
cin >> N;
ll ind = 1, dir = 1, cur = 1;
ll lo = 1, hi = N;
while (lo < hi) {
ll mid = (lo + hi) >> 1;
if (cur + dir * mid >= 1) {
cur += dir * mid;
} else {
ind += 1 - (cur + dir * mid);
cur = 1;
}
lo = mid + 1;
dir = -dir;
}
ask (ind);
lo = 1;
hi = N;
while (lo < hi) {
ll mid = (lo + hi) >> 1;
// cout << lo << " " << mid << " " << hi << " " << ind << "\n";
if (ind - mid >= 1) {
if (ask (ind - mid) == 1) {
hi = mid;
} else {
lo = mid + 1;
}
ind = ind - mid;
} else if (ind + mid <= N) {
if (ask (ind + mid) == 1) {
hi = mid;
} else {
lo = mid + 1;
}
ind = ind + mid;
} else {
if (ind - 1 >= N - ind) {
if (ask(1) == 1) {
hi = ind - 1;
} else {
lo = ind;
}
ind = 1;
} else {
if (ask(N) == 1) {
hi = N - ind;
} else {
lo = N - ind + 1;
}
ind = N;
}
}
}
cout << "= " << hi << endl;
}
