#include <bits/stdc++.h>
using namespace std;
int n, x, y, a, b;
int main() {
  cin >> n;
  if (n % 4 != 0) {
    printf("! -1\n");
    return 0;
  }
  printf("? 1\n");
  cin >> x;
  printf("? %d\n", 1 + n / 2);
  cin >> y;
  if (x == y) {
    printf("! %d\n", x);
    return 0;
  }
  int l = 2, r = n / 2;
  while (l <= r) {
    int m = (l + r) / 2;
    printf("? %d\n", m);
    cin >> a;
    printf("? %d\n", m + n / 2);
    cin >> b;
    if (a == b) {
      printf("! %d\n", m);
      return 0;
    } else if (x > y && a > b || x < y && a < b)
      l = m + 1;
    else
      r = m;
  }
}
