#include <iostream>
#include <string>
#include <algorithm>
#include <utility>
#include <cmath>
#include <cstdlib>
#include <map>
#include <set>
#include <queue>
#include <vector>
using namespace std;
#define MOD 1000000007

const int MAX = 1000000;

long long fac[MAX], finv[MAX], inv[MAX];

void COMinit(long long mod) {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++) {
        fac[i] = fac[i - 1] * i % mod;
        inv[i] = mod - inv[mod % i] * (mod / i) % mod;
        finv[i] = finv[i - 1] * inv[i] % mod;
    }
}

long long COM(long long n, long long k, long long mod) {
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

int main() {
    COMinit(MOD);
    long long N, M; cin >> N >> M;
    vector<long long> A(N);
    for (int i = 0; i < N; i++) cin >> A[i];
    long long ans = 0;
    for (long long i = 0; i < N; i++) ans += A[i] * COM(N + M - i - 1, M - 1, MOD) % MOD;
    cout << ans << endl;
}