#include "bits/stdc++.h"
#include "atcoder/all"
using namespace std;
using namespace atcoder;
using mint = modint1000000007;
const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	long long n, m; cin >> n >> m;
	vector<int>a(n + 1);
	rep(i, n - 1) cin >> a[i + 2];
	mint com = m % mod;
	mint ans = 0;
	rrep2(i, 2, n + 1) {
		ans += com * a[i];
		com *= (n - i + m + 1);
		com *= inv_mod(n - i + 1 + 1, mod);
	}
	cout << ans.val() << endl;
	return 0;
}