#include <bits/stdc++.h>
using namespace std;
char gc() {
  static char buf[100000], *p1 = buf, *p2 = buf;
  return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 100000, stdin), p1 == p2)
             ? EOF
             : *p1++;
}
long long read() {
  char ch = getchar();
  long long x = 0;
  int op = 1;
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') op = -1;
  for (; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
  return x * op;
}
int n;
int ask(int x, int y) {
  printf(y ? "! " : "? ");
  printf("%d\n", x);
  fflush(stdout);
  if (!y)
    return read();
  else
    exit(0);
}
int main() {
  n = read();
  if (n % 4 == 2) ask(-1, 1);
  int x = ask(1, 0), y = ask(1 + n / 2, 0);
  int l = 2, r = n / 2, mid;
  while (l <= r) {
    mid = l + r >> 1;
    int a = ask(mid, 0), b = ask(mid + n / 2, 0);
    if (a == b) ask(mid, 1);
    if ((a < b) ^ (x < y))
      r = mid - 1;
    else
      l = mid + 1;
  }
  ask(-1, 1);
  return 0;
}
