#include <bits/stdc++.h>
using namespace std;
int n;
int res1, res2;
bool sign(int x) { return x > 0; }
int main() {
  scanf("%d", &n);
  printf("? 1\n");
  fflush(stdout);
  scanf("%d", &res1);
  printf("? %d\n", 1 + n / 2);
  fflush(stdout);
  scanf("%d", &res2);
  if (res1 == res2) {
    printf("! 1\n");
    return 0;
  } else if (abs(res1 - res2) & 1) {
    printf("! -1\n");
    return 0;
  } else {
    bool f1 = sign(res2 - res1);
    bool f2;
    int l = 1, r = 1 + n / 2;
    int mid;
    while (l <= r) {
      mid = (l + r) >> 1;
      printf("? %d\n", mid);
      fflush(stdout);
      scanf("%d", &res1);
      printf("? %d\n", mid + n / 2);
      fflush(stdout);
      scanf("%d", &res2);
      if (res1 == res2) {
        printf("! %d\n", mid);
        return 0;
      } else {
        f2 = sign(res2 - res1);
        if (f1 == f2)
          l = mid;
        else
          r = mid;
      }
    }
  }
}
