#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for(long long i = 0; i < (long long)(n); i++)
#define repi(i,a,b) for(long long i = (long long)(a); i < (long long)(b); i++)
using ll = long long; using vll = vector<ll>; using vvll = vector<vll>; 
static const long long mo = 1e9+7;

int main(void) {
    ll n, c; cin >> n >> c;
    vll a(n); rep(i, a.size()) cin >> a[i];

    static vvll dp(c+1, vll(n, 0));
    rep(j, n) dp[0][j] = 1;

    rep(i, c) rep(j, n) repi(h, j, n) 
        (dp[i+1][h] += dp[i][j] * a[j]) %= mo;
    ll ret = dp[c][n-1];

    rep(i, n) {
        ll tmp = 1;
        rep(j, c) 
            (tmp *= a[i]) %= mo;
        (ret -= tmp) %= mo;
    }
    cout << (ret + mo) % mo << endl;
    return 0;
}