#include <bits/stdc++.h>
#include <sstream>

//#define int long long
#define endl '\n'
#define IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
typedef long long ll;

using namespace std;

const int N=1e6+50;
int sz[N];
signed main() {
    int n,x;
    cin>>n;
    int a;
    printf("? 1\n");
    cin>>a;
    int l=1,r=n;
    if(a==0) {
        printf("! 1");
        return 0;
    }
    l=2;
    while(l<r) {
        int m=((l+r)>>1)+1;
        printf("? %d\n",m);
        cin>>x;
        if(x==0 and a==1) {
            a=x;
            r=m-1;
            printf("? %d\n",l);
            cin>>x;
            if(a==x) {
                printf("! %d",l);
                return 0;
            }
            a=x;
            continue;
        }
        if(x==a) {
            printf("! %d",m);
            return 0;
        }
        else if(x==a+1) {
            l=m+1;
        }
        else if(x==a-1) {
            r=m-1;
        }
        a=x;
    }
    printf("! %d",l);
    return 0;
}