#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.);
const long long mod = 1000000007;
inline void prep() {
  cin.tie(0);
  cin.sync_with_stdio(0);
};
int main() {
  prep();
  int n;
  cin >> n;
  if ((n / 2) % 2 == 1) {
    cout << "! -1" << endl;
    fflush(stdout);
    return 0;
  }
  long long num1, num2;
  cout << "? 1" << endl;
  fflush(stdout);
  cin >> num1;
  cout << "? " << 1 + n / 2 << endl;
  fflush(stdout);
  cin >> num2;
  int l = 1;
  int r = n / 2;
  long long goal = abs(num1 - num2);
  int pos = (n / 2 - goal) / 2;
  int neg = n / 2 - (n / 2 - goal) / 2;
  if (pos > neg) {
    if (num2 < num1) {
      int temp = neg;
      neg = pos;
      pos = temp;
    }
  } else if (pos < neg) {
    if (num2 > num1) {
      int temp = neg;
      neg = pos;
      pos = temp;
    }
  } else {
    cout << "! 1" << endl;
    fflush(stdout);
    return 0;
  }
  int mid = (l + r) / 2;
  while (l <= r) {
    mid = (l + r) / 2;
    int num11, num22;
    cout << "? " << mid << endl;
    fflush(stdout);
    cin >> num11;
    cout << "? " << mid + n / 2 << endl;
    fflush(stdout);
    cin >> num22;
    long long goall = abs(num11 - num22);
    int npos = (n / 2 - goall) / 2;
    int nneg = n / 2 - (n / 2 - goall) / 2;
    if (npos > n) {
      if (num22 < num11) {
        int temp = nneg;
        nneg = npos;
        npos = temp;
      }
    } else if (npos < nneg) {
      if (num22 > num11) {
        int temp = nneg;
        nneg = npos;
        npos = temp;
      }
    } else {
      cout << "! " << mid << endl;
      fflush(stdout);
      return 0;
    }
    if (pos > neg) {
      if (npos < nneg) {
        r = mid - 1;
      } else {
        l = mid + 1;
      }
    } else {
      if (npos > nneg) {
        r = mid - 1;
      } else {
        l = mid + 1;
      }
    }
  }
  cout << "! -1" << endl;
  fflush(stdout);
  return 0;
}
