#include <bits/stdc++.h>
using namespace std;
const int N = (int)2e5 + 7;
const int inf = (int)1e9 + 7;
int ask(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &x);
  return x;
}
void finish(int x) {
  printf("! %d", x);
  exit(0);
}
int sign(int x) {
  if (x > 0)
    return 1;
  else
    return 0;
}
int main() {
  int n;
  scanf("%d", &n);
  n /= 2;
  int dif1 = ask(1) - ask(n + 1);
  if (dif1 & 1) finish(-1);
  if (!dif1) finish(1);
  int l = 2;
  int r = n;
  while (l <= r) {
    int mid = (l + r) >> 1;
    int dif2 = ask(mid) - ask(mid + n);
    if (!dif2) finish(mid);
    if (sign(dif1) != sign(dif2)) {
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
}
