#include <bits/stdc++.h>   using namespace std;   const int N = 5e3 + 10, mod = 1e9 + 7;   int fact[N], invfact[N];   inline int add(int x, int y) {     x += y;     if (x >= mod) x -= mod;     return x;   }   inline int sub(int x, int y) {     x -= y;     if (x < 0) x += mod;     return x;   }   inline int mul(int x, int y) { return (((long long)x) * y) % mod; }   inline int powr(int a, long long b) {     int x = 1 % mod;     while (b) {       if (b & 1) x = mul(x, a);       a = mul(a, a);       b >>= 1;     }     return x;   }   inline int inv(int a) { return powr(a, mod - 2); }   void pre() {     fact[0] = invfact[0] = 1;     for (int i = 1; i < N; i++) fact[i] = mul(i, fact[i - 1]);     invfact[N - 1] = inv(fact[N - 1]);     for (int i = N - 2; i >= 1; i--) invfact[i] = mul(invfact[i + 1], i + 1);     assert(invfact[1] == 1);   }   inline int C(int n, int k) {     if (n < k || k < 0) return 0;     return mul(fact[n], mul(invfact[k], invfact[n - k]));   }   int n, k;   int C2[N + 10], A[N], E[N];   inline int getC2(int i) {     if (i > k) return 0;     return C2[i];   }   int F[N][N];   int main() {     pre();     cin >> n >> k;     F[0][0] = 1;     for (int i = 1; i <= n; i++) {       F[i][0] = 1;       cin >> A[i];       for (int j = 1; j <= i; j++) {         F[i][j] = add(F[i - 1][j], mul(A[i], F[i - 1][j - 1]));       }     }     C2[0] = 1;     for (int i = 1; i <= n + 10; i++)       C2[i] = mul(C2[i - 1], mul(k - i + 1, inv(i)));     int invn = inv(n);     int _div = 1;     for (int i = 0; i <= n - 1; i++) {       E[i] = mul(getC2(i + 1), _div);       _div = mul(_div, invn);       E[i] = mul(E[i], fact[i]);     }     int ans = 0;     for (int i = 0; i <= n - 1; i++) {       ans = add(           ans,           mul(F[n][i], mul(n - i, mul(powr(mod - 1, n - 1 - i), E[n - 1 - i]))));     }     ans = mul(ans, invn);     cout << ans;   }   