#include <bits/stdc++.h>
using namespace std;
const int N = 100001;
const long long MOD = 1e9 + 7;
const double eps = 1e-9;
int n, a[N];
int ask(int x) {
  printf("? %d\n", x + 1);
  fflush(stdout);
  scanf("%d", &x);
  return x;
}
int main() {
  scanf("%d", &n);
  int r1 = ask(0), r2 = ask(n / 2);
  if (r1 == r2) {
    puts("! 1");
    return 0;
  } else if (abs(r1 - r2) & 1) {
    puts("! -1");
    return 0;
  }
  int v1 = r1 - r2, v2;
  int l = 1, r = n / 2 - 1, m;
  while (l <= r) {
    m = (l + r) / 2;
    r1 = ask(m), r2 = ask(m + n / 2);
    v2 = r1 - r2;
    if (r1 == r2) {
      printf("! %d\n", m + 1);
      return 0;
    }
    if ((v1 < 0 && v2 > 0) || (v1 > 0 && v2 < 0))
      r = m - 1;
    else
      l = m + 1;
  }
  puts("! -1");
  return 0;
}
