#include <bits/stdc++.h>
using namespace std;
const long long N = 2e5 + 10;
const long long P = 1000 * 1000 * 1000 + 7;
long long n, k, sum, ans;
long long fac[N], a[N];
long long chert(long long x, long long y) {
  if (y == 0) {
    return 1;
  }
  long long ret = chert(x, y / 2);
  ret *= ret;
  ret %= P;
  if (y % 2) {
    ret *= x;
    ret %= P;
  }
  return ret;
}
long long ch(long long x, long long y) {
  if (x < y) {
    return 0;
  }
  long long ret = fac[x] * chert(fac[y], P - 2);
  ret %= P;
  ret *= chert(fac[x - y], P - 2);
  ret %= P;
  return ret;
}
long long f(long long n, long long k) {
  long long ret = 0;
  for (long long i = 1; i <= k; i++) {
    long long x = 1ll * ch(k, i) * chert(i, n);
    x %= P;
    if (i % 2 == k % 2)
      ret += x;
    else
      ret -= x - P;
  }
  ret %= P;
  return ret;
}
void set_fac() {
  fac[0] = 1;
  for (long long i = 1; i <= n; i++) {
    fac[i] = fac[i - 1] * i;
    fac[i] %= P;
  }
}
void read_input() {
  cin >> n >> k;
  for (long long i = 1; i <= n; i++) {
    cin >> a[i];
    sum += a[i];
  }
  sum %= P;
}
void solve() {
  set_fac();
  ans = f(n, k) + 1ll * f(n - 1, k) * (n - 1);
  ans %= P;
  ans *= sum;
  ans %= P;
  ans *= chert(fac[k], P - 2);
  ans %= P;
}
void write_output() { cout << ans << endl; }
int32_t main() {
  ios ::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  read_input(), solve(), write_output();
  return 0;
}
