#pragma GCC optimize ("O2")
#pragma GCC target ("avx2")
#include<bits/stdc++.h>
//#include<atcoder/all>
//using namespace atcoder;

using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define Would
#define you
#define please

const int mod = 1e9 + 7;
constexpr ll modpow(ll A, ll B) {
	ll kotae = 1;
	while (B > 0) {
		if (B & 1) kotae = kotae * A % mod;
		A = A * A % mod;
		B >>= 1;
	}
	return kotae;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	ll N, M;
	cin >> N >> M;
	M %= mod;
	int A[100100];
	rep(i, N - 1) cin >> A[N - i - 2];

	ll kotae = 0;
	ll kari = 1;
	rep(i, N - 1) {
		kari = kari * (M + i) % mod;
		kari = kari * modpow(i + 1, mod - 2) % mod;
		kotae += kari * A[i] % mod;
	}

	co(kotae% mod);

	Would you please return 0;
}