#include <bits/stdc++.h>
using namespace std;
int n, m, num[100100];
inline int as(int u) {
  int a, b;
  if (num[u]) return num[u];
  printf("? %d\n", u);
  fflush(stdout);
  scanf("%d", &a);
  return num[u] = a;
}
inline void pd(int u) {
  if (as(u) == as(u + m)) {
    printf("! %d\n", u);
    exit(0);
  }
}
int main() {
  int i, j, l, r, mid, p, q;
  cin >> n;
  m = n / 2;
  if (m % 2) {
    puts("! -1");
    return 0;
  }
  as(1), as(m + 1), pd(1);
  l = 2, r = m;
  for (; l <= r;) {
    mid = ((l + r) >> 1);
    p = as(mid), q = as(mid + m);
    pd(mid);
    if ((p > q) ^ (num[1] > num[m + 1]))
      r = mid - 1;
    else
      l = mid + 1;
  }
}
