#include<bits/stdc++.h>
using namespace std;
int n;

void init() {
   cin >> n;
}

int ask(int x) {
   cout << "? " << x << "\n";
   cout << flush;
   int a; cin >> a;
   return a;
}

void fav(int ans) {
   cout << "! " << ans << "\n";
   exit(0);
}

void range(int f1, int l1, int f2, int l2, int diff, int out, bool type) {
   if(f1 > l1 || f2 > l2 || f1 > l2) assert(0);
   if(type) {
      int mid = f1 + l2 >> 1;
      int val = ask(mid);
      if(abs(out) == val) fav(mid);

      if(out > 0) {
         if(val > out) range(f1, 0, 0, mid - 1, 0, val, 1);
         else range(mid + 1, 0, 0, l2, 0, val, 1);
      }
      else if(out < 0) {
         if(val > - out) range(mid + 1, 0, 0, l2, 0, val, 1);
         else range(f1, 0, 0, mid - 1, 0, val, 1);
      }
      else range(f1, mid - 1, mid + 1, l2, val, out, 0);
   }
   else {
      if(l1 - f1 + 1 >= l2 - f2 + 1) {
         int mid = l1 + f1 >> 1;
         int val = ask(mid);
         if(diff == val) fav(mid);

         if(diff > val) range(mid + 1, 0, 0, l1, 0, out + val, 1);
         else if(diff < val) range(f1, mid - 1, f2, l2, val, out, 0);
      }
      else {
         int mid = l2 + f2 >> 1;
         int val = ask(mid);
         if(diff == val) fav(mid);

         if(diff > val) range(f2, 0, 0, mid - 1, 0, out - val, 1);
         else if(diff < val) range(f1, l1, mid + 1, l2, val, out, 0);
      }
   }
}

int main () {
   init();

   range(1, 0, 0, n, 0, 0, 1);
}