#include <bits/stdc++.h>
#include <atcoder/modint>
using mint = atcoder::modint1000000007;
using ll = long long;
using namespace std;

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

    int n;
    ll m;
    cin >> n >> m;
    n--;

    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    mint r = 0, t = 1;
    for (int i = 0; i < n; i++) {
        t *= m + i;
        t /= i + 1;
        r += t * a[n - 1 - i];
    }

    cout << r.val() << endl;

    return 0;
}