#include <iostream>

using namespace std;

const int INF = 1e9;

int main()
{
    int n, ans, l, r;

    cin >> n;

    l = 0, r = n;
    while( l < r - 1 ) {
      int mij = ( l + r ) / 2;
      cout << "? " << mij;
      cout.flush();

      cin >> ans;
      if( ans == - 1 )
        break;
      else if( ans == 0 )
        l = mij;
      else
        r = mij;
    }

    cout << "! " << r;
    cout.flush();

    return 0;
}