#include <bits/stdc++.h>
using namespace std;
int n;
int A[200005];
int vt[200005];
int test(int x) {
  int k = n / 2;
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &A[x]);
  printf("? %d\n", x + k);
  fflush(stdout);
  scanf("%d", &A[x + k]);
  return A[x + k] - A[x];
}
int main() {
  scanf("%d", &n);
  int k = n / 2;
  if (k % 2) {
    puts("! -1");
    fflush(stdout);
    return 0;
  }
  int l = 1, r = k + 1;
  vt[l] = test(l);
  vt[r] = -vt[l];
  int ans = -1;
  int mid = 1;
  while (vt[mid] != 0) {
    mid = (l + r) / 2;
    int d = vt[mid] = test(mid);
    if (d == 0) {
      ans = mid;
      break;
    }
    if (d * vt[l] < 0) {
      r = mid;
    } else {
      l = mid;
    }
  }
  if (ans == -1 && vt[l] != 0 && vt[r] != 0) {
    puts("! -1");
    fflush(stdout);
    return 0;
  }
  ans = mid;
  printf("! %d\n", ans);
  fflush(stdout);
  return 0;
}
