#include <bits/stdc++.h>
using namespace std;
int n;
int l, r;
void did(int &x, int y) {
  printf("? %d\n", y);
  fflush(stdout);
  scanf("%d", &x);
}
int main() {
  scanf("%d", &n);
  if ((n / 2) & 1) {
    printf("! -1");
    fflush(stdout);
    return 0;
  }
  while (1) {
    printf("? 1\n");
    fflush(stdout);
    int x, y;
    int l = 1, r = n / 2;
    scanf("%d", &x);
    printf("? %d\n", n / 2 + 1);
    fflush(stdout);
    scanf("%d", &y);
    if (x == y) {
      printf("! %d\n", 1);
      fflush(stdout);
      return 0;
    }
    int xx = x, yy = y;
    while (l <= r) {
      int mid = (l + r) / 2;
      did(xx, mid);
      did(yy, mid + n / 2);
      if (xx == yy) {
        printf("! %d\n", mid);
        fflush(stdout);
        return 0;
      }
      if ((xx < yy) == (x < y))
        l = mid + 1;
      else
        r = mid - 1;
    }
  }
  return 0;
}
