#include <bits/stdc++.h>
using namespace std;
int n;
int ask(int x) {
  int num1, num2;
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &num1);
  printf("? %d\n", x + n / 2);
  fflush(stdout);
  scanf("%d", &num2);
  if (num1 == num2) {
    printf("! %d\n", x);
    fflush(stdout);
    exit(0);
  }
  return num1 < num2;
}
void solve() {
  scanf("%d", &n);
  int type = ask(1);
  int L = 1, R = n / 2 + 1;
  while (L + 1 < R) {
    int mid = (L + R) >> 1;
    if (ask(mid) == type)
      L = mid;
    else
      R = mid;
  }
  puts("! -1");
  fflush(stdout);
}
int main() {
  solve();
  return 0;
}
