#include <bits/stdc++.h>
using namespace std;
char getc() {
  char c = getchar();
  while ((c < 'A' || c > 'Z') && (c < 'a' || c > 'z') && (c < '0' || c > '9'))
    c = getchar();
  return c;
}
int gcd(int n, int m) { return m == 0 ? n : gcd(m, n % m); }
int read() {
  int x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    x = (x << 1) + (x << 3) + (c ^ 48), c = getchar();
  return x * f;
}
int n, k, a[5010], f[5010][5010], ans;
int ksm(int a, int k) {
  int s = 1;
  for (; k; k >>= 1, a = 1ll * a * a % 1000000007)
    if (k & 1) s = 1ll * s * a % 1000000007;
  return s;
}
int inv(int a) { return ksm(a, 1000000007 - 2); }
void inc(int &x, int y) {
  x += y;
  if (x >= 1000000007) x -= 1000000007;
}
signed main() {
  const char LL[] = "%lld\n";
  n = read(), k = read();
  for (int i = 1; i <= n; i++) a[i] = read();
  f[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    f[i][0] = 1ll * f[i - 1][0] * a[i] % 1000000007;
    for (int j = 1; j <= i; j++)
      f[i][j] = (1ll * f[i - 1][j] * a[i] - f[i - 1][j - 1] + 1000000007) %
                1000000007;
  }
  int s = 1;
  for (int i = 0; i <= min(k, n); i++) {
    inc(ans, 1ll * ksm(n, k - i) * f[n][i] % 1000000007 * s % 1000000007);
    s = 1ll * s * (k - i) % 1000000007;
  }
  ans = 1ll * ans * inv(ksm(n, k)) % 1000000007;
  ans = (f[n][0] - ans + 1000000007) % 1000000007;
  cout << ans;
  return 0;
}
