#include <bits/stdc++.h>
using namespace std;
#define int long long
#define db long double
#define hset unordered_set
int n;
bool qry(int x) {
cout << "? " << x << endl;
bool ans; cin >> ans;
return ans;
}
void brt() {
int ps = 1, drc = +1, dst = n - 1; qry(ps);
while (true) {
int nw_ps = ps + drc * dst;
if (ps == nw_ps || !qry(nw_ps)) { cout << "= " << dst + 1 << endl; return; }
ps = nw_ps, drc *= -1, dst--;
}
assert(false);
}
bool chck(int st) {
int ps = st, drc = -1;
int lw = 1, hg = n;
while (lw != hg) {
// cout << lw << " " << hg << ": " << ps << endl;
int md = (lw + hg) / 2;
ps += drc * md;
if (ps < 1 || n < ps) return false;
lw = md + 1;
drc *= -1;
}
return true;
}
int gt_st() {
int st = roundl(n * (db) 2 / 3);
for (int i = st - 100; i <= st + 100; i++)
if (chck(i)) return i;
assert(false);
}
hset<int> vs;
bool vld(int ps) {
return (!vs.count(ps) && 1 <= ps && ps <= n);
}
int bn_srch(int st) {
int ps = st, drc = -1;
int lw = 1, hg = n;
qry(ps), vs.insert(ps);
while (lw != hg) {
int md = (lw + hg) / 2;
if (!vld(ps + drc * md)) {
while (lw <= md && !vld(ps + drc * md)) md--;
if (!vld(ps + drc * md)) md = (lw + hg) / 2;
}
if (!vld(ps + drc * md)) {
while (!vld(ps + drc * md) || vs.count(ps)) {
ps -= drc;
}
qry(ps), vs.insert(ps);
}
ps = ps + drc * md;
if (qry(ps)) hg = md;
else lw = md + 1;
vs.insert(ps);
drc *= -1;
}
return lw;
}
signed main() {
cin >> n;
if (n <= 64) { brt(); exit(0); }
int st = gt_st();
int ans = bn_srch(st);
cout << "= " << ans << endl;
}
