#include <bits/stdc++.h>
using namespace std;
const double pi = 3.141592653589;
long long int mod = 998244353;
long long int mod2 = 1000000009;
const long long int inf = 1e9;
const long long int inff = 9000000000000000000;
long double eps = 1e-9;
const long long int base1 = 304933;
const long long int base2 = 4395853;
long double power(long long int x, long long int y, long long int p) {
  long long int res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long int modInverse(long long int n, long long int p) {
  return power(n, p - 2, p);
}
long long int fac[1];
long long int invfac[1];
long long int ncr(long long int n, long long int r, long long int p) {
  if (r > n) return 0;
  if (r == 0) return 1;
  return (fac[n] * modInverse(fac[r], p) % p * modInverse(fac[n - r], p) % p) %
         p;
}
long long int log(long long int a, long long int b) {
  if (a == 1) return b;
  long long int ans = 0;
  while (b) {
    ans++;
    b /= a;
  }
  return ans - 1;
}
int is_prime[1];
int smallestfactor[1];
void sieve() {
  for (long long int i = 2; i <= 5000000; i++) {
    if (is_prime[i]) {
      for (long long int j = i * i; j <= 5000000; j += i) {
        if (is_prime[j] == 1) {
          is_prime[j] = 0;
          smallestfactor[j] = i;
        }
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int test = 1;
  while (test--) {
    int n;
    cin >> n;
    int l = 1, r = 1 + n / 2, m, a, b, x;
    cout << "? " << l;
    cout << "\n";
    cout.flush();
    cin >> a;
    cout << "? " << r;
    cout << "\n";
    cout.flush();
    cin >> b;
    if (a == b) {
      cout << "! 1\n";
      return 0;
    }
    x = a - b;
    while (r - l > 1) {
      m = (l + r) / 2;
      cout << "? " << m;
      cout << "\n";
      cout.flush();
      cin >> a;
      cout << "? " << m + n / 2;
      cout << "\n";
      cout.flush();
      cin >> b;
      if (a == b) {
        cout << "! " << m << "\n";
        return 0;
      }
      if ((a - b) * (x) < 0) {
        r = m;
      } else {
        l = m;
      }
    }
    cout << "? " << l;
    cout << "\n";
    cout.flush();
    cin >> a;
    cout << "? " << l + n / 2;
    cout << "\n";
    cout.flush();
    cin >> b;
    if (a == b) {
      cout << "! " << l;
      cout << "\n";
      return 0;
    }
    cout << "? " << r;
    cout << "\n";
    cout.flush();
    cin >> a;
    cout << "? " << r + n / 2;
    cout << "\n";
    cout.flush();
    cin >> b;
    if (a == b) {
      cout << "! " << r;
      cout << "\n";
      return 0;
    }
    cout << "! -1\n";
    cout.flush();
  }
  return 0;
}
