#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000010;
int a[maxn], f[maxn];
char chr = '?';
int get(int x) {
  cout << chr << ' ' << x << endl;
  int num;
  cin >> num;
  return num;
}
int main() {
  int N, i, j;
  scanf("%d", &N);
  a[1] = get(1);
  a[1 + N / 2] = get(1 + N / 2);
  f[1] = a[1] - a[1 + N / 2];
  if (f[1] == 0) {
    cout << '!' << " 1" << endl;
    return 0;
  }
  int L = 1, R = N / 2;
  while (L <= R) {
    int Mid = (L + R) / 2;
    a[Mid] = get(Mid);
    a[Mid + N / 2] = get(Mid + N / 2);
    f[Mid] = a[Mid] - a[Mid + N / 2];
    if (f[Mid] == 0) {
      cout << '!' << " " << Mid << endl;
      return 0;
    }
    if (f[Mid] * f[1] < 0)
      R = Mid - 1;
    else
      L = Mid + 1;
  }
  cout << '!' << " -1" << endl;
  return 0;
}
