#include <bits/stdc++.h>
using namespace std;
int a[100005], n, x, minn = 3;
bool m1[100005], m2[100005];
int read() {
  int w = 0, f = 1;
  char c = getchar();
  while (c > '9' || c < '0') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    w = (w << 3) + (w << 1) + (c ^ 48);
    c = getchar();
  }
  return w * f;
}
int main() {
  n = read(), x = read();
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
  for (int i = 1; i <= n; i++) {
    if (m1[a[i]]) minn = min(0, minn);
    if (m2[a[i]]) minn = min(1, minn);
    if (m1[a[i] & x]) minn = min(1, minn);
    if (m2[a[i] & x]) minn = min(2, minn);
    m1[a[i]] = 1, m2[a[i] & x] = 1;
  }
  if (minn == 3)
    puts("-1");
  else
    printf("%d\n", minn);
  return 0;
}
