#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  long long x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    x = (x << 1) + (x << 3) + c - '0';
    c = getchar();
  }
  return x * f;
}
int n;
inline int Ask(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  return read();
}
void Output(int x) {
  printf("! %d\n", x);
  fflush(stdout);
  exit(0);
}
inline bool In(int l1, int r1, int l2, int r2) {
  if (l1 > r1) swap(l1, r1);
  if (l2 > r2) swap(l2, r2);
  if (l2 > r1) return false;
  if (l1 > r2) return false;
  return true;
}
int main() {
  n = read();
  if (n & 2) {
    Output(-1);
  }
  int A = Ask(1), B = Ask(1 + (n >> 1)), l = 1, r = (n >> 1) + 1;
  if (A == B) {
    Output(1);
  }
  bool t = (A < B);
  while (l <= r) {
    int mid = (l + r) >> 1;
    int p = Ask(mid), q = Ask(mid + (n >> 1));
    int d = (p < q);
    if (p == q) {
      Output(mid);
    }
    if (d == t) {
      l = mid + 1;
    } else {
      r = mid - 1;
    }
  }
  return 0;
}
