#include <atcoder/modint>
#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;
using lint = long long;
using mint = atcoder::modint1000000007;

void solve() {
    int n;
    lint m;
    cin >> n >> m;
    --m;

    vector<int> xs(n - 1);
    for (auto& x : xs) cin >> x;
    reverse(xs.begin(), xs.end());

    mint c = 1, ans = 0;
    for (int i = 1; i <= n - 1; ++i) {
        c = c * (m + i) / i;
        ans += c * xs[i - 1];
    }

    cout << ans.val() << "\n";
}

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

    solve();

    return 0;
}