#include <bits/stdc++.h>
using namespace std;
namespace FullPre {
template <typename T>
inline T read() {
  T f = 1, x = 0;
  char c = getchar();
  while (!isdigit(c)) {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (isdigit(c)) {
    x = x * 10 + c - '0';
    c = getchar();
  }
  return x * f;
}
template <typename T>
inline void write(T x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
template <typename T>
inline void print_char(T x, char ch) {
  write(x), putchar(ch);
}
template <typename T>
inline void char_print(char ch, T x) {
  putchar(ch);
  write(x);
}
template <typename T>
inline void print_space(T x) {
  print_char(x, ' ');
}
template <typename T>
inline void space_print(T x) {
  char_print(' ', x);
}
template <typename T>
inline void println(T x) {
  print_char(x, '\n');
}
template <typename T>
inline void lnprint(T x) {
  char_print('\n', x);
}
template <typename T>
inline void print_array_in_range1(T* a, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) print_char(a[i], " \n"[i == r]);
}
template <typename T>
inline void print_array_in_range2(T* a, int l, int r) {
  println(r - l + 1), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range3(T* a, int l, int r) {
  for (int(i) = (r); (i) >= (l); --(i)) print_char(a[i], " \n"[i == l]);
}
template <typename T>
inline void print_array_in_range4(T* a, int l, int r) {
  println(r - l + 1), print_array_in_range3(a, l, r);
}
template <typename T>
inline void print_array_in_range5(T* a, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) println(a[i]);
}
template <typename T>
inline void print_array_in_range6(T* a, int l, int r) {
  println(r - l + 1), print_array_in_range5(a, l, r);
}
template <typename T>
inline void print_array_in_range7(T* a, int l, int r) {
  for (int(i) = (r); (i) >= (l); --(i)) println(a[i]);
}
template <typename T>
inline void print_array_in_range8(T* a, int l, int r) {
  println(r - l + 1), print_array_in_range7(a, l, r);
}
template <typename T>
inline void print_array_in_range9(T* a, int l, int r) {
  print_space(r - l + 1), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range10(T* a, int l, int r) {
  print_space(r - l + 1), print_array_in_range3(a, l, r);
}
template <typename T>
inline void print_array_in_range11(T* a, int l, int r) {
  print_space(r - l + 1), print_array_in_range5(a, l, r);
}
template <typename T>
inline void print_array_in_range12(T* a, int l, int r) {
  print_space(r - l + 1), print_array_in_range7(a, l, r);
}
template <typename T>
inline void print_array_in_range13(T* a, int l, int r, int n) {
  println(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range14(T* a, int l, int r, int n) {
  println(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range15(T* a, int l, int r, int n) {
  println(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range16(T* a, int l, int r, int n) {
  println(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range17(T* a, int l, int r, int n) {
  print_space(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range18(T* a, int l, int r, int n) {
  print_space(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range19(T* a, int l, int r, int n) {
  print_space(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array_in_range20(T* a, int l, int r, int n) {
  print_space(n), print_array_in_range1(a, l, r);
}
template <typename T>
inline void print_array1(T* a, int n) {
  print_array_in_range1(a, 1, n);
}
template <typename T>
inline void print_array2(T* a, int n) {
  println(n), print_array1(a, n);
}
template <typename T>
inline void print_array3(T* a, int n) {
  print_array_in_range3(a, 1, n);
}
template <typename T>
inline void print_array4(T* a, int n) {
  println(n), print_array3(a, n);
}
template <typename T>
inline void print_array5(T* a, int n) {
  print_array_in_range5(a, 1, n);
}
template <typename T>
inline void print_array6(T* a, int n) {
  println(n), print_array5(a, n);
}
template <typename T>
inline void print_array7(T* a, int n) {
  print_array_in_range7(a, 1, n);
}
template <typename T>
inline void print_array8(T* a, int n) {
  println(n), print_array7(a, n);
}
template <typename T>
inline void print_array9(T* a, int n) {
  print_space(n), print_array1(a, n);
}
template <typename T>
inline void print_array10(T* a, int n) {
  print_space(n), print_array3(a, n);
}
template <typename T>
inline void print_array11(T* a, int n) {
  print_space(n), print_array5(a, n);
}
template <typename T>
inline void print_array12(T* a, int n) {
  print_space(n), print_array7(a, n);
}
template <typename T>
inline void vector_print(vector<T> a, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) print_char(a[i], " \n"[i == r]);
}
template <typename T>
inline void map_print(map<T, T> a, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) print_char(a[i], " \n"[i == r]);
}
template <typename T>
inline void stack_print(stack<T> a, char ch) {
  while (!a.empty()) print_char(a.top(), ch), a.pop();
}
template <typename T>
inline void queue_print(queue<T> a, char ch) {
  while (!a.empty()) print_char(a.front(), ch), a.pop();
}
template <typename T>
inline void pq_print(priority_queue<T> a, char ch) {
  while (!a.empty()) print_char(a.top(), ch), a.pop();
}
inline void print_string_in_range(string s, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) putchar(s[i]);
}
inline void print_chars_in_range(char* a, int l, int r) {
  for (int(i) = (l); (i) <= (r); ++(i)) putchar(a[i]);
}
inline void print_allstring(string s) {
  print_string_in_range(s, 0, (int)s.size() - 1);
}
inline void filein(string s) { freopen((s + ".in").c_str(), "r", stdin); }
inline void fileout(string s) { freopen((s + ".out").c_str(), "w", stdout); }
inline void file(string s) { filein(s), fileout(s); }
template <typename T>
inline T mymin(T a, T b) {
  return a < b ? a : b;
}
template <typename T>
inline T mymax(T a, T b) {
  return a > b ? a : b;
}
template <typename T>
inline T myabs(T a) {
  return a < 0 ? -a : a;
}
template <typename T>
inline T mysq(T a) {
  return a * a;
}
template <typename T>
inline T mygcd(T a, T b) {
  return !b ? a : mygcd(b, a % b);
}
template <typename T>
inline T mylcm(T a, T b) {
  return a / mygcd(a, b) * b;
}
}  // namespace FullPre
using namespace FullPre;
namespace Solution {
const int N = 2e5 + 7, mod = 1e9 + 7;
int n, k, sum, w[N], fac[N], inv[N];
inline int ksm(int a, int b) {
  int res = 1;
  for (; b; b >>= 1, a = 1ll * a * a % mod)
    if (b & 1) res = 1ll * res * a % mod;
  return res;
}
inline int S2(int n, int m) {
  int res = 0;
  for (int(i) = (0); (i) <= (m); ++(i))
    res = (res + 1ll * ((i & 1) ? mod - 1 : 1) * ksm(m - i, n) % mod * inv[i] %
                     mod * inv[m - i] % mod) %
          mod;
  return res;
}
inline void Main() {
  n = read<int>(), k = read<int>(), fac[0] = 1;
  for (int(i) = (1); (i) <= (n); ++(i))
    w[i] = read<int>(), sum = (sum + w[i]) % mod,
    fac[i] = 1ll * fac[i - 1] * i % mod;
  inv[n] = ksm(fac[n], mod - 2);
  for (int(i) = (n); (i) >= (1); --(i)) inv[i - 1] = 1ll * inv[i] * i % mod;
  return println(1ll * sum * (S2(n, k) + 1ll * (n - 1) * S2(n - 1, k) % mod) %
                 mod),
         void();
}
}  // namespace Solution
int main() {
  Solution ::Main();
  return 0;
}
