import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int n = Integer.parseInt(sa[0]);
		long m = Long.parseLong(sa[1]);
		sa = br.readLine().split(" ");
		int[] a = new int[n];
		for (int i = 0; i < n - 1; i++) {
			a[i + 1] = Integer.parseInt(sa[i]);
		}
		br.close();

		int mod = 1000000007;
		long ans = 0;
		long val = 1;
		long m2 = m;
		for (int i = 1; i < n; i++) {
			val = val * (m2 % mod) % mod;
			val = val * modinv(i, mod) % mod;
			ans += val * a[n - i] % mod;
			m2++;
		}
		ans %= mod;
		System.out.println(ans);
	}

	static long modinv(long a, int m) {
		long b = m;
		long u = 1;
		long v = 0;
		long tmp = 0;

		while (b > 0) {
			long t = a / b;
			a -= t * b;
			tmp = a;
			a = b;
			b = tmp;

			u -= t * v;
			tmp = u;
			u = v;
			v = tmp;
		}

		u %= m;
		if (u < 0) u += m;
		return u;
	}
}