#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 7;
int n, k, a[5007], c[N], r[N];
int solve() {
  sort(a, a + n);
  for (int i = (0); i < (n); ++i)
    for (int j = (i + 1); j < (n); ++j) ++c[a[j] - a[i]];
  int lim = k * (k + 1) >> 1, timeStamp = 0;
  for (int m = (1); m < (N); ++m) {
    int sum = 0;
    for (int i = m; i < N && sum <= lim; i += m) sum += c[i];
    if (sum > lim) continue;
    ++timeStamp, sum = 0;
    for (int i = (0); i < (n); ++i)
      if (r[a[i] % m] != timeStamp) {
        r[a[i] % m] = timeStamp;
      } else {
        ++sum;
        if (sum > k) break;
      }
    if (sum <= k) return m;
  }
  return -1;
}
int main() {
  scanf("%d%d", &n, &k);
  for (int i = (0); i < (n); ++i) scanf("%d", &a[i]);
  printf("%d", solve());
  return 0;
}
