#include <bits/stdc++.h>
using namespace std;
void getint(int &v) {
  char ch, fu = 0;
  for (ch = '*'; (ch < '0' || ch > '9') && ch != '-'; ch = getchar())
    ;
  if (ch == '-') fu = 1, ch = getchar();
  for (v = 0; ch >= '0' && ch <= '9'; ch = getchar()) v = v * 10 + ch - '0';
  if (fu) v = -v;
}
const int MO = 1e9 + 7;
long long P[5555], ans, p[5555], T;
int n, k, a[5555], f[5010][5010];
long long pw(long long x, long long y) {
  long long t = 1;
  for (; y; y >>= 1) {
    if (y & 1) t = t * x % MO;
    x = x * x % MO;
  }
  return t;
}
int main() {
  cin >> n >> k;
  T = 1;
  for (int i = 1; i <= n; ++i) getint(a[i]), T = T * a[i] % MO;
  P[0] = 1;
  for (int i = 1; i <= n; ++i) P[i] = P[i - 1] * (k - i + 1) % MO;
  f[0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    f[i][0] = 1;
    for (int j = 1; j <= i; ++j)
      f[i][j] = (f[i - 1][j] + (long long)f[i - 1][j - 1] * a[i]) % MO;
  }
  long long t = MO - pw(n, MO - 2);
  p[0] = 1;
  for (int i = 1; i <= n; ++i) p[i] = p[i - 1] * t % MO;
  for (int i = 0; i <= n; ++i) {
    if (n - i > k) continue;
    long long x = f[n][i] * P[n - i] % MO * p[n - i] % MO;
    (ans += x) %= MO;
  }
  cout << (T - ans + MO) % MO << endl;
  return 0;
}
