#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const int maxn = 200010;
long long sum, n, k, w;
long long f[maxn], inf[maxn];
long long p(long long a, long long n) {
  if (!~a) return n & 1 ? -1 : 1;
  long long b = 1;
  while (n) {
    if (n & 1) (b *= a) %= mod;
    (a *= a) %= mod;
    n >>= 1;
  }
  return b;
}
long long c(long long n, long long k) {
  if (k > n || n < 0) return 0;
  if (k == 0 || n == k) return 1;
  return f[n] * inf[k] % mod * inf[n - k] % mod;
}
long long stirling(long long n, long long k) {
  long long ret = 0;
  for (long long j = 0; j <= k; j++)
    (ret += p(-1, k + j) * c(k, j) % mod * p(j, n) % mod) %= mod;
  return ret * inf[k] % mod;
}
int main() {
  ios::sync_with_stdio(false);
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> w;
    (sum += w) %= mod;
  }
  f[0] = inf[0] = 1;
  for (int i = 1; i <= n; i++) {
    f[i] = f[i - 1] * i % mod;
    inf[i] = inf[i - 1] * p(i, mod - 2) % mod;
  }
  long long coe1 = stirling(n, k) % mod;
  long long coe2 = stirling(n - 1, k) * (n - 1) % mod;
  cout << ((coe1 + coe2 + mod) % mod * sum % mod + mod) % mod << endl;
  return 0;
}
