#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, ans;
int last;
map<int, int> used;
int get(int p) {
used[p] = 1;
cout << "? " << p << endl;
int res;
cin >> res;
if(last && res && abs(last - p) < ans) {
ans = abs(last - p);
}
last = p;
return res;
}
int f(int x) {
int i = n - x;
return 1 + i / 2;
}
void Solve() {
cin >> n;
ans = n;
int l = 1, r = n / 2 - (n % 2 == 0);
while(l <= r) {
int mid = (l + r) / 2;
int a = f(mid + mid), b = f(mid + mid) + mid + mid;
get(a);
if(get(b)) {
ans = min(mid + mid + 1, ans);
r = mid - 1;
}
else {
l = mid + 1;
}
}
used[last] = 0;
for(int i = 1; i + ans - 1 <= n && ans % 2 == 0 && ans > 1; i++) {
if(!used[i] && !used[i + ans - 1]) {
used[last] = 1;
int a = i, b = i + ans - 1;
if(b == last) {
swap(a, b);
}
if(a != last) {
get(a);
}
get(b);
break;
}
}
cout << "= " << ans << endl;
}
//
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
Solve();
return 0;
}
