#include <bits/stdc++.h>
using namespace std;
int N;
int arr[100005];
bool asked[100005];
int ask(int a) {
  cout << "? " << a << endl;
  fflush(stdout);
  int res;
  cin >> res;
  return res;
}
void answer(int a) {
  cout << "! " << a << endl;
  fflush(stdout);
  return;
}
int askPair(int a) { return ask(a) - ask(a + N / 2); }
int main() {
  cin >> N;
  if (N & 2) {
    answer(-1);
    return 0;
  }
  int l = 1;
  int r = 1 + N / 2;
  arr[l] = askPair(1);
  arr[r] = -arr[l];
  if (arr[l] == 0) {
    answer(l);
    return 0;
  }
  while (l < r) {
    int m = (l + r + 1) / 2;
    if (asked[m]) {
      answer(-1);
      return 0;
    }
    asked[m] = true;
    arr[m] = askPair(m);
    if (arr[m] == 0) {
      answer(m);
      return 0;
    }
    if (abs(arr[m]) / arr[m] == abs(arr[l]) / arr[l]) {
      l = m;
    } else {
      r = m;
    }
  }
  answer(-1);
  return 0;
}
