#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const int MAXN = 100005;
int n;
int sgn(int x) {
  if (x < 0) return -1;
  if (x > 0) return 1;
  return 0;
}
int mem[MAXN];
int diff(int x) {
  int &ans = mem[x];
  int a, b;
  if (ans != INF) return ans;
  cout << "? " << x << '\n';
  fflush(stdout);
  cin >> a;
  int y = x + n / 2;
  cout << "? " << y << '\n';
  fflush(stdout);
  cin >> b;
  ans = b - a;
  mem[y] = -ans;
  if (ans == 0) {
    cout << "! " << x << '\n';
    exit(0);
  }
  return ans;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) mem[i] = INF;
  if (n % 4 != 0) {
    puts("! -1");
    return 0;
  }
  int l = 1, r = n / 2 + 1;
  diff(l);
  while (l != r) {
    int mid = (l + r) >> 1;
    diff(mid);
    if (sgn(diff(mid)) == sgn(diff(l)))
      l = mid;
    else
      r = mid;
  }
  puts("! -1");
  return 0;
}
