#include <bits/stdc++.h>
using namespace std;
const int P = 1e9 + 7, INF = 0x3f3f3f3f;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long qpow(long long a, long long n) {
  long long r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1) r = r * a % P;
  return r;
}
long long inv(long long first) {
  return first <= 1 ? 1 : inv(P % first) * (P - P / first) % P;
}
inline int rd() {
  int first = 0;
  char p = getchar();
  while (p < '0' || p > '9') p = getchar();
  while (p >= '0' && p <= '9') first = first * 10 + p - '0', p = getchar();
  return first;
}
const int N = 1e6 + 10;
int n, k, a[N], c[N], tmp[N];
int main() {
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; ++i) scanf("%d", a + i);
  c[0] = a[1], c[1] = -1;
  for (int i = 2; i <= n; ++i) {
    for (int j = 0; j <= n; ++j) tmp[j] = (long long)c[j] * a[i] % P;
    for (int j = 0; j <= n; ++j) tmp[j + 1] = (tmp[j + 1] - c[j]) % P;
    for (int j = 0; j <= n; ++j) c[j] = tmp[j];
  }
  int ans = 1;
  for (int i = 1; i <= n; ++i) ans = (long long)ans * a[i] % P;
  int now = k, invn = inv(n), ret = 1;
  for (int i = 0; i <= min(n, k); ++i) {
    ans = (ans - (long long)ret * c[i]) % P;
    ret = (long long)ret * now % P * invn % P;
    --now;
  }
  if (ans < 0) ans += P;
  printf("%d\n", ans);
}
