#include <iostream>
using namespace std;

constexpr int M = 1000000007;

long long modinv(long long a, long long m) {
    long long x = m, y = a, p = 1, q = 0, r = 0, s = 1;
    while (y != 0) {
        long long u = x / y;
        long long x0 = y; y = x - y * u; x = x0;
        long long r0 = p - r * u, s0 = q - s * u;
        p = r; r = r0; q = s; s = s0;
    }
    return q < 0 ? q + m : q;
}

int main() {
    long long n, m; cin >> n >> m;
    int a[n-1]; for (int i = 0; i < n-1; i++) cin >> a[i];
    long long ans = 0, c = m % M;
    for (int i = n-2; i >= 0; i--) {
        ans += a[i] * c % M;
        cerr << ans << ' ' << c <<  endl;
        c = (n + m - i - 1) % M * modinv(n - i, M) % M * c % M;
    }
    cout << ans % M << endl;
}