#include<bits/stdc++.h>

#define int long long

using namespace std;
const int m=41;
const int N=1e6+10;
bool light[N];

signed main()
{	
	int n;
	cin>>n;
	int num[m];
	int sb[n+1];
	for(int i=0;i<n+1;i++) sb[i]=0;
	sb[1]=1;
	for(int i=1;i<m;i++)
	{
		cout<<"? "<<sb[i]<<endl;
		scanf("%d",&num[i]);
		
		if(num[1]==0)
		{
			cout<<"! "<<sb[i]<<endl;
			cout.flush();
			break;
		}
		else if(num[1]==1)//右
			sb[i+1]=(sb[i]+n)/2;
		else if(num[1]==-1)//左
			sb[i+1]=(sb[i]+1)/2;
		
		if(i>=2)
		{
			if(num[i]==num[i-1])
			{
				cout<<"! "<<sb[i]<<endl;
				cout.flush();
				break;
			}
			else if(num[i]==num[i-1]+1)
			{
				sb[i+1]=(sb[i]+n)/2+1;
			}
			else if(num[i]==num[i-1]-1)
			{
				sb[i+1]=(sb[i]+sb[i-1])/2-1;
			}
		}
	}
	return 0;
}