#include <bits/stdc++.h>
using namespace std;
inline int gi() {
  int data = 0, m = 1;
  char ch = 0;
  while (ch != '-' && (ch < '0' || ch > '9')) ch = getchar();
  if (ch == '-') {
    m = 0;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    data = (data << 1) + (data << 3) + (ch ^ 48);
    ch = getchar();
  }
  return (m) ? data : -data;
}
int n, K, N, Sum, Ans, IFac[2000005], inv[2000005];
inline int Pow(int ts, int js) {
  int al = 1;
  while (js) {
    if (js & 1) al = 1ll * al * ts % 1000000007;
    ts = 1ll * ts * ts % 1000000007;
    js >>= 1;
  }
  return al;
}
int main() {
  n = gi();
  K = gi();
  int Sum = 0;
  for (int i = 1; i <= n; i++) Sum = (Sum + gi()) % 1000000007;
  if (n == 1 && K == 1) return cout << Sum << endl, 0;
  N = max(N, K);
  inv[0] = inv[1] = IFac[0] = IFac[1] = 1;
  for (int i = 2; i <= N; i++)
    inv[i] =
        1ll * (1000000007 - 1000000007 / i) * inv[1000000007 % i] % 1000000007,
    IFac[i] = 1ll * IFac[i - 1] * inv[i] % 1000000007;
  Ans = 0;
  for (int j = 0; j <= K - 1; j++) {
    int ret = 0;
    ret = 1ll * (n - 1) * Pow(K - j, n - 2) % 1000000007;
    ret = (ret + Pow(K - j, n - 1)) % 1000000007;
    Ans = (Ans + 1ll * ((j & 1) ? 1000000007 - 1 : 1) * IFac[j] % 1000000007 *
                     IFac[K - j - 1] % 1000000007 * ret % 1000000007) %
          1000000007;
  }
  Ans = 1ll * Ans * Sum % 1000000007;
  cout << Ans << endl;
  return 0;
}
