#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  bool neg = false;
  cout << "? 1" << endl;
  int x;
  cin >> x;
  cout << "? " << 1 + n / 2 << endl;
  int y;
  cin >> y;
  if (abs(x - y) & 1) {
    cout << "! -1" << endl;
    return 0;
  }
  if (x > y)
    neg = false;
  else if (x < y)
    neg = true;
  else if (x == y) {
    cout << "! 1" << endl;
    return 0;
  }
  for (int md, l = 2, r = n / 2; l <= r;) {
    md = (l + r) / 2;
    bool isneg = false;
    int x, y;
    cout << "? " << md << endl;
    cin >> x;
    cout << "? " << md + n / 2 << endl;
    cin >> y;
    if (x > y)
      isneg = false;
    else if (x < y)
      isneg = true;
    else if (x == y) {
      cout << "! " << md << endl;
      return 0;
    }
    if (isneg == neg)
      l = md + 1;
    else
      r = md - 1;
  }
  return 0;
}
