#include<bits/stdc++.h>
using namespace std;
#define x first
#define y second
typedef long long LL;
const int N=1e3+10,M=510;
int n,m;
pair<int,int>a[N],b[N];
bool flag;
int query(int x){
    int cur;
    cout<<"? "<<x<<endl;
    cout.flush();
    cin>>cur;
    return cur;
}

void dfs(int l,int r,int x){
    int cur=l+r>>1;
    int y=query(cur);
    if(y==abs(x)) {
        cout<<"! "<<cur<<endl;
        flag=true;
        return;
    }
    if(x==0){
        if(flag) return;
        dfs(cur+1,r,-1);
        if(flag) return;
        dfs(l,cur,1);
    }else if(x<0){
        if(y>abs(x)) dfs(cur+1,r,-y);
        else dfs(l,cur,-y);
    }else{
        if(y>abs(x)) dfs(l,cur,y);
        else dfs(cur+1,r,y);
    }
}
int main() {
    scanf("%d",&n);
    int l=1,r=n;
    dfs(1,n,0);


}