#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  char ch = getchar();
  long long s = 0, w = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') w = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    s = s * 10 + ch - '0';
    ch = getchar();
  }
  return s * w;
}
long long n, k;
long long w[200010];
long long val1, val2;
long long fac[200010], ans, inv[200010];
inline long long ksm(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % 1000000007;
    b >>= 1;
    a = a * a % 1000000007;
  }
  return ans;
}
inline long long C(long long n, long long m) {
  return fac[n] * inv[m] % 1000000007 * inv[n - m] % 1000000007;
}
inline long long STL2(long long n, long long m) {
  long long sum = 0, c = 1;
  for (register long long i = 0; i <= m; i++) {
    sum = (sum + c * C(m, i) * ksm(m - i, n) % 1000000007) % 1000000007;
    c = -c;
  }
  return sum * inv[m] % 1000000007;
}
int main() {
  n = read(), k = read();
  for (register long long i = 1; i <= n; i++) w[i] = read();
  fac[0] = 1;
  inv[0] = inv[1] = 1;
  for (register long long i = 1; i <= n; i++)
    fac[i] = fac[i - 1] * i % 1000000007;
  for (register long long i = 2; i <= n; i++)
    inv[i] = inv[1000000007 % i] * (1000000007 - 1000000007 / i) % 1000000007;
  for (register long long i = 2; i <= n; i++)
    inv[i] = inv[i] * inv[i - 1] % 1000000007;
  val1 = STL2(n - 1, k), val2 = STL2(n, k);
  val1 = val1 * (n - 1) % 1000000007;
  for (register long long i = 1; i <= n; i++)
    ans = (ans + w[i] * (val1 + val2) % 1000000007) % 1000000007;
  printf("%lld\n", (ans + 1000000007) % 1000000007);
  return 0;
}
