#include <stdio.h>

const int Mod = 1000000007;

long long div_mod(long long x, long long y, long long z)
{
	if (x % y == 0) return x / y;
	else return (div_mod((1 + x / y) * y - x, (z % y), y) * z + x) / y;
}

int main()
{
	int i, N, A[100001];
	long long M;
	scanf("%d %lld", &N, &M);
	for (i = 2; i <= N; i++) scanf("%d", &(A[i]));
	M %= Mod;
	
	long long tmp = M - 1, tmpp = M - 1, ans = 0;
	for (i = N; i >= 2; i--) {
		ans += A[i] * (tmpp + 1) % Mod;
		tmp = tmp * (M - i + N) % Mod * div_mod(1, N - i + 2, Mod) % Mod;
		tmpp += tmp;
		if (tmpp >= Mod) tmpp -= Mod;
	}
	printf("%lld\n", ans % Mod);
	fflush(stdout);
	return 0;
}