#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) {a = max(a, b);}
template<class T> void chmin(T& a, const T& b) {a = min(a, b);}
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using mint = modint1000000007;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    ll m;
    cin >> n >> m;
    VI a(n);
    a[0] = 0;
    for(int i = 1; i < n; i++) cin >> a[i];
    mint now = 1;
    mint ans;
    rep(i, n - 1) {
        now *= mint(m + i);
        now /= 1 + i;
        ans += a[n - 1 - i] * now;
    }
    cout << ans.val() << '\n';
}