#pragma GCC optimize("Ofast","unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define int long long
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
vector<bool> tested(N+1);
auto optTest = [&](int x)->bool{
if(tested[x]){
cerr<<"CALLED DOUBLE QUERY!!!\n";
return 1;
}
tested[x]=true;
cout << "? " << x << endl;
int c;cin>>c;
return c==1;
};
int offset=0;
auto getAtPos = [&](int idx){
if(offset)idx=N-idx+1ll;
int center = N/2ll;
if(N&1ll)center++;
int dist = abs(center-idx);
if(idx<=center)return 1ll+2ll*dist;
else return 2ll*dist;
};
auto getWhichPos = [&](int x){
int center = N/2ll;
if(N&1ll)center++;
int dist = x/2ll;
if(x&1)return center-dist;
else return center+dist;
};
if(N<=60){ // bruteforce
optTest(getWhichPos(1));
for(int i=2;i<=60;i++){
if(optTest(getWhichPos(i))){
cout << "= " << i-1 << endl;
return 0;
}
}
cout << "= " << N << endl;
return 0;
}
}
