#include <bits/stdc++.h>
using namespace std;
int n;
int interactive(int x) {
  int a, b;
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &a);
  printf("? %d\n", x + n / 2);
  fflush(stdout);
  scanf("%d", &b);
  return a - b;
}
int main() {
  scanf("%d", &n);
  if (n % 4 != 0) {
    printf("! -1\n");
    fflush(stdout);
  } else {
    int fl = interactive(1), fr;
    if (fl == 0) {
      printf("! %d\n", 1);
      fflush(stdout);
    } else {
      fr = -fl;
      int l = 1, r = n / 2 + 1;
      while (l <= r) {
        int mid = (l + r) >> 1;
        int g = interactive(mid);
        if (g == 0) {
          printf("! %d\n", mid);
          fflush(stdout);
          return 0;
        }
        if ((g < 0) == (fl < 0))
          l = mid;
        else
          r = mid;
      }
    }
  }
  return 0;
}
