#include <bits/stdc++.h>
using namespace std;
const int maxn = 200000;
const int mod = 1e9 + 7;
const long double Pi = acos(-1.0);
const double eps = 1e-7;
template <class T>
int read(T &x) {
  x = 0;
  int f = 0;
  char ch = getchar();
  while (!isdigit(ch)) {
    f |= (ch == '-');
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  x = f ? -x : x;
  return x;
}
int n, k, w, sum, ans, num;
int fac[maxn + 5], invf[maxn + 5];
inline int inc(int a, int b) { return a + b >= mod ? a + b - mod : a + b; }
inline int power(int a, int b, int p) {
  int ans = 1;
  for (; b; b >>= 1) {
    if (b & 1) ans = 1ll * ans * a % p;
    a = 1ll * a * a % p;
  }
  return ans;
}
inline int S(int n, int m) {
  int sum = 0;
  for (int i = 0; i <= m; i++) {
    int num = 1ll * power(i, n, mod) * invf[m - i] % mod * invf[i] % mod;
    if ((m - i) & 1) num = mod - num;
    sum = inc(sum, num);
  }
  return sum;
}
int main() {
  read(n);
  read(k);
  fac[0] = invf[0] = 1;
  for (int i = 1; i <= n; i++) sum = inc(sum, read(w));
  int maxx = max(n, k);
  for (int i = 1; i <= maxx; i++) fac[i] = 1ll * fac[i - 1] * i % mod;
  invf[maxx] = power(fac[maxx], mod - 2, mod);
  for (int i = maxx - 1; i >= 1; i--)
    invf[i] = 1ll * invf[i + 1] * (i + 1) % mod;
  ans = 1ll * sum * inc(S(n, k), 1ll * (n - 1) * S(n - 1, k) % mod) % mod;
  printf("%d", ans);
  return 0;
}
