#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
int n, k, w[200010], fac[200010] = {1}, inv[200010] = {1};
int modpow(int a, long long n, int mod = MOD) {
  int r = 1, b = a;
  while (n > 0) {
    if (n & 1) r = 1ll * r * b % mod;
    b = 1ll * b * b % mod;
    n >>= 1;
  }
  return r;
}
long long nCk(int n, int k) {
  if (k < 0 || k > n) return 0;
  return 1ll * fac[n] * inv[k] % MOD * inv[n - k] % MOD;
}
long long nSk(int n, int k) {
  if (n == 0 && k == 0) return 1;
  if (k < 0 || k > n) return 0;
  long long res = 0;
  for (int i = 0; i <= k; i++) {
    int v = (k + i) % 2 ? -1 : 1;
    res = (res + v * nCk(k, i) * modpow(i, n) % MOD + MOD) % MOD;
  }
  res = res * inv[k] % MOD;
  return res;
}
int main() {
  for (int i = 1; i <= 200000; i++) fac[i] = 1ll * i * fac[i - 1] % MOD;
  for (int i = 1; i <= 200000; i++) inv[i] = modpow(fac[i], MOD - 2);
  scanf("%d%d", &n, &k);
  long long sum = 0;
  for (int i = 1; i <= n; i++) scanf("%d", w + i), sum += w[i];
  sum %= MOD;
  printf("%lld", (nSk(n, k) + (n - 1) * nSk(n - 1, k)) % MOD * sum % MOD);
  return 0;
}
