#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n % 4 == 2) {
    printf("! %d", -1);
  } else {
    printf("? %d\n", 1);
    fflush(stdout);
    int resp1;
    cin >> resp1;
    printf("? %d\n", (n / 2) + 1);
    fflush(stdout);
    int resp2;
    cin >> resp2;
    if (resp1 == resp2) {
      printf("! %d", 1);
      return 0;
    }
    bool smaller = (resp1 > resp2);
    int lo = 1;
    int hi = n / 2;
    while (true) {
      int mid = (lo + hi) / 2;
      printf("? %d\n", mid);
      fflush(stdout);
      cin >> resp1;
      printf("? %d\n", (n / 2) + mid);
      fflush(stdout);
      cin >> resp2;
      if (resp1 == resp2) {
        printf("! %d", mid);
        break;
      }
      bool flag = (resp1 > resp2);
      if (smaller == flag) {
        lo = mid + 1;
      } else {
        hi = mid - 1;
      }
    }
  }
}
