mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    N, M = map(int, input().split())
    A = list(map(int, input().split()))
    N -= 2
    M -= 2

    fac = [1] * (N+3)
    finv = [1] * (N+3)
    for i in range(1, N+3):
        finv[i] = (finv[i-1] * pow(i, mod-2, mod))%mod
        fac[i] = (fac[i-1] * (M+i))%mod

    def comb(i, j):
        return (fac[i - M] * finv[j])%mod

    ans = 0
    for i, a in enumerate(A):
        ans += (a * ((((N + 2 - i) * comb(M+(N-i)+2, (N-i)+2))%mod - M - 1) * pow(M+1, mod-2, mod)))%mod
        ans = (ans + a)%mod
    print(ans)


if __name__ == '__main__':
    main()