#include <bits/stdc++.h>
using namespace std;
int n;
const int MAX_N = 100024;
long long f[MAX_N];
void in(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  int m, y;
  scanf("%d", &m);
  printf("? %d\n", x + n / 2);
  fflush(stdout);
  scanf("%d", &y);
  f[x] = y - m;
}
int main() {
  scanf("%d", &n);
  in(1);
  int l = 1, r = n / 2;
  if (f[1] == 0) {
    printf("! 1\n");
    fflush(stdout);
    return 0;
  }
  while (l <= r) {
    int mid = (l + r) >> 1;
    in(mid);
    if (f[mid] == 0) {
      printf("! %d\n", mid);
      fflush(stdout);
      return 0;
    } else if (f[mid] > 0) {
      if (f[1] > 0) {
        l = mid + 1;
      } else
        r = mid - 1;
    } else {
      if (f[1] > 0) {
        r = mid - 1;
      } else
        l = mid + 1;
    }
  }
  printf("! -1\n");
  fflush(stdout);
  return 0;
}
