#include <bits/stdc++.h>
using namespace std;
long long n;
long long f(long long x) {
  cout << "? " << x << endl;
  long long ans;
  cin >> ans;
  return ans;
}
long long check(long long x) {
  long long A = x, B = x + n / 2;
  if (B > n) B -= n;
  return f(A) - f(B);
}
void report(long long x) {
  cout << "! " << x << endl;
  exit(0);
}
int main() {
  cin >> n;
  if (n % 4) {
    report(-1);
  }
  int l = 1, r = n;
  long long x = check(1);
  if (x == 0) {
    report(1);
  }
  bool is_pos = x > 0;
  while (r >= l) {
    int m = l + r >> 1;
    long long x = check(m);
    if (x == 0) {
      report(m);
    }
    if (is_pos ^ (x > 0)) {
      l = m;
    } else {
      r = m;
    }
  }
  assert(false);
}
