coci15c5p3.cpp

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

const ll MOD = 1e9 + 7;

const int N = 100005;
const int K = 55;

ll a[N], mem[N][K];

ll comb(ll n, ll k) {
  if (k < 0 || n < 0 || k > n) return 0;
  if (k == 1) return n;
  if (k == 0 || k == n) return 1;
  if (mem[n][k] != -1) return mem[n][k];
  return mem[n][k] = (comb(n - 1, k) + comb(n - 1, k - 1)) % MOD;
}

signed main() {
  cin.tie(0)->sync_with_stdio(0);
  memset(mem, -1, sizeof(mem));
  int n, k;
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort(a + 1, a + n + 1);
  ll ans = 0, cnt = 0;
  for (int i = 1; i <= n; i++) {
    if (i > 1 && a[i] != a[i - 1]) cnt = 0;
    cnt++;
    if (i < n && a[i] == a[i + 1]) continue;
    for (int j = 1; j <= cnt; j++) {
      ans += ((comb(cnt, j) * comb(i - cnt, k - j)) % MOD * a[i]) % MOD;
      ans %= MOD;
    }
  }
  cout << ans << '\n';
}