#include <bits/stdc++.h>
using namespace std;
const int SIZE = 1 << 10;
int pointer = SIZE;
char buffer[SIZE];
char Advance() {
  if (pointer == SIZE) {
    fread(buffer, 1, SIZE, stdin);
    pointer = 0;
  }
  return buffer[pointer++];
}
int Read() {
  int answer = 0;
  char ch = Advance();
  while (!isdigit(ch)) ch = Advance();
  while (isdigit(ch)) {
    answer = answer * 10 + ch - '0';
    ch = Advance();
  }
  return answer;
}
const int MAXN = 200000;
const int MOD = 1000000007;
int v[1 + MAXN];
void Add(int &x, int y) {
  x += y;
  if (x >= MOD) x -= MOD;
}
void Subtract(int &x, int y) {
  x -= y;
  if (x < 0) x += MOD;
}
int factorial[1 + MAXN], inverse[1 + MAXN];
int RaiseToPower(int base, int power) {
  int answer = 1;
  while (power) {
    if (power % 2) answer = (1LL * answer * base) % MOD;
    base = (1LL * base * base) % MOD;
    power /= 2;
  }
  return answer;
}
void Precompute(int n) {
  factorial[0] = inverse[0] = 1;
  for (int i = 1; i <= n; i++)
    factorial[i] = (1LL * factorial[i - 1] * i) % MOD;
  inverse[n] = RaiseToPower(factorial[n], MOD - 2);
  for (int i = n - 1; i >= 1; i--)
    inverse[i] = (1LL * (i + 1) * inverse[i + 1]) % MOD;
}
int Combinations(int n, int k) {
  int answer = (1LL * inverse[n - k] * inverse[k]) % MOD;
  answer = (1LL * answer * factorial[n]) % MOD;
  return answer;
}
int Stirling(int n, int k) {
  int answer = 0, sign = 1;
  for (int i = k; i >= 0; i--) {
    int add = (1LL * Combinations(k, i) * RaiseToPower(i, n)) % MOD;
    if (sign)
      Add(answer, add);
    else
      Subtract(answer, add);
    sign = 1 - sign;
  }
  answer = (1LL * answer * inverse[k]) % MOD;
  return answer;
}
int main() {
  int n, k, sum = 0;
  scanf("%d%d", &n, &k);
  Precompute(n);
  for (int i = 1; i <= n; i++) {
    int x;
    scanf("%d", &x);
    Add(sum, x);
  }
  if (k == n) {
    printf("%d\n", sum);
    return 0;
  }
  if (k == 1) {
    sum = (1LL * sum * n) % MOD;
    printf("%d\n", sum);
    return 0;
  }
  int first = Stirling(n, k),
      second = (1LL * (n - 1) * Stirling(n - 1, k)) % MOD;
  sum = (1LL * sum * (first + second)) % MOD;
  printf("%d\n", sum);
  return 0;
}
