#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const int MAXN = 1e6 + 5;

bool vis[MAXN];

int ask(int x)
{
	vis[x] = 1;
	printf("? %d\n",x);
	fflush(stdout);
	int res;
	scanf("%d",&res);
	return res;
}

void answer(int x)
{
	printf("! %d\n",x);
	fflush(stdout);
	exit(0);
}

int main(void)
{
	int n;
	scanf("%d",&n);
	
	int lst = ask(1);
	if(lst == 0) answer(1);
	
	int pos = 1;
	
	int l = 2, r = n;
	while(l < r)
	{
		int mid = (l+r)>>1;
		int cur = ask(mid);
		if(cur == lst) answer(mid);
		
		if(cur > lst) r = mid-1;
		else l = mid+1;
		
		if(l == r) answer(l);
		
		lst = cur;
		if(lst == 0)
		{
			while(vis[pos]) ++pos;
			cur = ask(pos);
			if(cur == lst) answer(pos);
			lst = cur;
			while(vis[l]) ++l;
		}
	}
	
	answer(l);
	return 0;
}