#include <bits/stdc++.h>
using namespace std;
const int MOD = int(1e9) + 7;
inline int norm(int a) {
  while (a >= MOD) a -= MOD;
  while (a < 0) a += MOD;
  return a;
}
inline int mul(int a, int b) { return int((a * 1ll * b) % MOD); }
inline int binPow(int a, int k) {
  int ans = 1;
  while (k > 0) {
    if (k & 1) ans = mul(ans, a);
    a = mul(a, a);
    k >>= 1;
  }
  return ans;
}
const int N = 200 * 1000 + 555;
int f[N], inf[N];
void precalc() {
  f[0] = inf[0] = 1;
  for (int i = int(1); i < int(N); i++) {
    f[i] = mul(f[i - 1], i);
    inf[i] = mul(inf[i - 1], binPow(i, MOD - 2));
  }
}
int n, k, w[N];
inline bool read() {
  if (!(cin >> n >> k)) return false;
  for (int i = int(0); i < int(n); i++) cin >> w[i];
  return true;
}
inline int c(int n, int k) {
  if (k > n || n < 0) return 0;
  if (k == 0 || n == k) return 1;
  return mul(f[n], mul(inf[n - k], inf[k]));
}
inline int s(int n, int k) {
  if (n == 0) return k == 0;
  if (k == 0) return n == 0;
  int ans = 0, sg[2] = {1, MOD - 1};
  for (int cnt = int(0); cnt < int(k); cnt++)
    ans = norm(ans + mul(sg[cnt & 1], mul(c(k, cnt), binPow(k - cnt, n))));
  return mul(ans, inf[k]);
}
inline void solve() {
  int sum = 0;
  for (int i = int(0); i < int(n); i++) sum = norm(sum + w[i]);
  int s0 = s(n, k);
  int s1 = mul(n - 1, s(n - 1, k));
  cout << mul(sum, norm(s0 + s1)) << endl;
}
int main() {
  precalc();
  assert(read());
  solve();
  return 0;
}
