#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 7;
const double EPS = 1e-9;
const double PI = 3.14159265359;
void ask(long long a) { cout << "? " << a << endl; }
void res(long long a) { cout << "! " << a << endl; }
int sign(long long a) {
  if (a > 0) return 1;
  return -1;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<long long> b(n / 2 + 1);
  long long a1, a2;
  ask(1);
  cin >> a1;
  ask(n / 2 + 1);
  cin >> a2;
  b[0] = a1 - a2;
  b[n / 2] = -b[0];
  if (b[0] % 2) {
    res(-1);
    return 0;
  }
  if (b[0] == 0) {
    res(1);
    return 0;
  }
  int l = 0, r = n / 2;
  while (r - l > 1) {
    int m = (r + l) / 2;
    ask(m + 1);
    cin >> a1;
    ask(m + 1 + n / 2);
    cin >> a2;
    b[m] = a1 - a2;
    if (b[m] == 0) {
      res(m + 1);
      return 0;
    }
    if (sign(b[m]) != sign(b[0])) {
      r = m;
    } else {
      l = m;
    }
  }
  if (b[l] == 0) {
    res(l + 1);
  } else {
    res(r + 1);
  }
  return 0;
}
