#include <bits/stdc++.h>
using namespace std;
#ifndef LOCAL
bool qry(int x){
cout << "? " << x << endl;
bool r; cin >> r;
return r;
}
void answer(int x){
cout << "= " << x << endl;
exit(0);
}
#else
int prv = -1, N, C;
bool qry(int x){
assert(1 <= x && x <= N);
if(prv == -1) { prv = x; return 0; }
bool res = abs(x - prv) >= C; prv = x;
return res;
}
void answer(int x){
assert(x == C);
}
#endif
void solve(int N){
int lo = 1, hi = N;
while(lo != hi){
int mid = (lo + hi) / 2;
qry(1);
if(qry(mid + 1)){
hi = mid;
}else{
lo = mid + 1;
}
}
cout << lo << endl;
answer(lo);
}
int32_t main(){
#ifdef LOCAL
random_device rd;
mt19937 rng(rd());
const int MX = 10;
for(int t = 0; t < 1000; t++){
N = rng() % MX + 1;
C = rng() % N + 1;
solve(N);
}
#else
int N; cin >> N;
solve(N);
#endif
}
