#include <bits/stdc++.h>
using namespace std;
const int maxn = 200007;
const int mod = 1e9 + 7;
long long pown(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return ans;
}
int add(int x, int y) { return (x + y) % mod; }
int mul(int x, int y) { return 1LL * x * y % mod; }
int inv[maxn], fac[maxn];
int C(int a, int b) {
  if (a < b) return 0;
  return mul(fac[a], mul(inv[b], inv[a - b]));
}
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') x = x * 10 + ch - '0', ch = getchar();
  return x * f;
}
int a[maxn];
int main() {
  fac[0] = inv[0] = 1;
  int n = read(), K = read();
  int tota = 0;
  for (int i = 1; i <= n; i++) {
    a[i] = read();
    tota = add(tota, a[i]);
  }
  for (int i = 1; i <= K; i++) fac[i] = 1LL * fac[i - 1] * i % mod;
  inv[K] = pown(fac[K], mod - 2);
  for (int i = K - 1; i; i--) inv[i] = mul(inv[i + 1], i + 1);
  int ans = 0;
  if (n == 1) {
    if (K == 1)
      printf("%d", tota);
    else
      printf("0");
    return 0;
  }
  for (int i = 0; i <= K - 1; i++) {
    int x = pown(mod - 1, i);
    x = mul(x, mul(inv[i], inv[K - 1 - i]));
    int y = K - 1 - i;
    int r = (mul(n, pown(y + 1, n - 1)) -
             mul(mul(n - 1, y), pown(y + 1, n - 2)) + mod) %
            mod;
    r = mul(r, x);
    ans = add(r, ans);
  }
  ans = mul(ans, tota);
  cout << ans;
  return 0;
}
