#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double eps = 1e-9;
const int inf = 2000000000;
const long long infLL = 9000000000000000000;
inline bool checkBit(long long n, int i) { return n & (1LL << i); }
inline long long setBit(long long n, int i) {
  return n | (1LL << i);
  ;
}
inline long long resetBit(long long n, int i) { return n & (~(1LL << i)); }
int dx[] = {0, 0, +1, -1};
int dy[] = {+1, -1, 0, 0};
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
inline bool isLeapYear(long long year) {
  return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}
inline void normal(long long &a) {
  a %= 1000000007;
  (a < 0) && (a += 1000000007);
}
inline long long modMul(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a * b) % 1000000007;
}
inline long long modAdd(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a + b) % 1000000007;
}
inline long long modSub(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline long long modPow(long long b, long long p) {
  long long r = 1;
  while (p) {
    if (p & 1) r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline long long modInverse(long long a) { return modPow(a, 1000000007 - 2); }
inline long long modDiv(long long a, long long b) {
  return modMul(a, modInverse(b));
}
bool seive[55];
int prime;
void seiveGen(int limit) {
  limit += 100;
  int sqrtn = sqrt(limit);
  for (int i = 3; i <= sqrtn; i += 2) {
    if (!seive[i >> 1]) {
      for (int j = i * i; j < limit; j += i + i) {
        seive[j >> 1] = 1;
      }
    }
  }
  prime++;
  for (int i = 3; i < limit; i += 2) {
    if (!seive[i >> 1]) prime++;
  }
  cout << prime << '\n';
}
int n, a[3005];
int ask(int mid) {
  int x;
  cout << "? " << mid << '\n';
  fflush(stdout);
  cin >> x;
  return x;
}
int main() {
  cin >> n;
  if (n % 4) {
    cout << "! " << -1 << '\n';
    fflush(stdout);
    return 0;
  }
  int lo = 1, hi = n / 2, mid, x, y, dif, val1 = ask(1) - ask(n / 2 + 1);
  while (lo <= hi) {
    mid = (lo + hi) >> 1;
    x = ask(mid);
    y = ask(mid + n / 2);
    dif = x - y;
    if (x == y) {
      cout << "! " << mid << '\n';
      fflush(stdout);
      return 0;
    } else if ((dif >= 0 && val1 >= 0) || (dif < 0 && val1 < 0))
      lo = mid + 1, val1 = dif;
    else
      hi = mid - 1;
  }
  cout << "! " << -1 << '\n';
  fflush(stdout);
  return 0;
}
