#include<bits/stdc++.h>
#define int long long
#define F first
#define S second
#define pb push_back
const int mod=1e9+7;
const int N=200000+10;
using namespace std;//初始化
typedef pair <int, int> pi;
int n,anss;
void digui(int l,int r,int zhi){
	int mid=(l+r)/2;
	cout<<"? "<<mid<<endl;
	int now,noww;
	cin>>now;
	if(now==zhi){
		anss=mid;
		return ;
	}
	cout<<"? "<<l<<endl;
	cin>>noww;
	if(now==noww){
		anss=l;
		return ;
		
	}
	else{
		if(noww>now){
			digui(mid+1,r,noww);
		}
		else if(noww<now){
			digui(l+1,mid,noww);
		}
	}
	return;
}
void solve(){
	cin>>n;
	digui(1,n,0);
	cout<<"! "<<anss<<endl;
}
signed main(){
	ios::sync_with_stdio(false);cin.tie(0); cout.tie(0);
	cout << fixed << setprecision(12);//小数保留位数
	int t=1;
//	cin>>t;
	while(t--){
		solve();
	}
	return 0;
}