#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int MN = 200010;
int exp(int x, int n) {
  int ret = 1;
  while (n) {
    if (n & 1) ret = 1LL * ret * x % mod;
    x = 1LL * x * x % mod;
    n >>= 1;
  }
  return ret;
}
int inv(int x) { return exp(x, mod - 2); }
int fact[MN], invf[MN];
int comb(int n, int k) {
  if (n < k) return 0;
  return 1LL * fact[n] * invf[k] % mod * invf[n - k] % mod;
}
int S(int n, int k) {
  int ret = 0;
  for (int i = 0; i < k; i++) {
    ret += 1LL * comb(k, i) * (i % 2 ? mod - 1 : 1) % mod * exp(k - i, n) % mod;
    ret %= mod;
  }
  ret = 1LL * ret * invf[k] % mod;
  return ret;
}
int N, K;
int W[MN];
int main() {
  fact[0] = 1;
  for (int i = 1; i < MN; i++) {
    fact[i] = 1LL * fact[i - 1] * i % mod;
  }
  for (int i = 0; i < MN; i++) {
    invf[i] = inv(fact[i]);
  }
  scanf("%d %d", &N, &K);
  int sum = 0;
  for (int i = 0; i < N; i++) {
    scanf("%d", &W[i]);
    sum += W[i];
    sum %= mod;
  }
  int ans =
      1LL * ((S(N, K) + 1LL * (N - 1) * S(N - 1, K) % mod) % mod) * sum % mod;
  printf("%d", ans);
}
