#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e6 + 100;
const int inf = 0x3f3f3f3f;
const int iinf = 1 << 30;
const long long linf = 2e18;
const long long mod = 1e9 + 7;
const double eps = 1e-7;
long long power(long long a, long long b) {
  long long ret = 1;
  for (; b; b >>= 1, (a *= a) %= mod)
    if (b & 1) (ret *= a) %= mod;
  return ret;
}
long long n, m;
long long a[maxn], fac[maxn], inv[maxn];
long long s1, s2, ans;
void init() {
  fac[0] = 1;
  for (register int i = (1); i <= (n); ++i) fac[i] = fac[i - 1] * i % mod;
  inv[n] = power(fac[n], mod - 2);
  for (register int i = (n); i >= (1); --i) inv[i - 1] = inv[i] * i % mod;
}
signed main() {
  scanf("%lld %lld", &n, &m);
  init();
  for (register int i = (1); i <= (n); ++i) scanf("%lld", &a[i]);
  for (register int i = (0); i <= (m); ++i) {
    (s1 += power(mod - 1, i) * power(m - i, n) % mod * inv[i] % mod *
           inv[m - i] % mod) %= mod;
    (s2 += power(mod - 1, i) * power(m - i, n - 1) % mod * inv[i] % mod *
           inv[m - i] % mod) %= mod;
  }
  for (register int i = (1); i <= (n); ++i)
    (ans += a[i] * (s1 + (n - 1) * s2 % mod) % mod) %= mod;
  printf("%lld\n", ans);
  return 0;
}
