n,c = map(int,input().split())
a = list(map(int,input().split()))

from collections import defaultdict
d = defaultdict(int)
for i in a:
    d[i] += 1
P = 10 ** 9 + 7
def f(c,k):
    if k > c:return 0
    c -= 1
    k -= 1
    ans = 1
    for i in range(k):
        ans = ans * (c - i) * pow(k - i,P-2,P) % P
    return ans
    
ans = 0
for i in d:
    inv = pow(i,P-2,P)
    u = 1
    for j in d:
        if j == i:continue
        t = (1 - j * inv) % P
        t = pow(t,P-2,P)
        t = pow(t,d[j],P)
        u = u * t % P
    v = pow(i,c,P)
    ans += (u * f(c,d[i]) - d[i]) * v
    ans %= P
print(ans)