import sys

MOD = 1000000007


def prod(a, b):
    return 0 if a < b else fac[a] * finv[a-b]


def mod_inverse(a):
    r, w = [1, a], [0, MOD]
    while w[1] is not 1:
        q = r[1] // w[1]
        nw = [r[0]-q*w[0], r[1]-q*w[1]]
        r = w
        w = nw
    return (MOD + w[0] % MOD) % MOD


if __name__ == '__main__':
    n, B = map(int, sys.stdin.readline().split())
    cnt, inv, fac, finv = [0]*(n+1), [0]*(n+1), [0]*(n+1), [0]*(n+1)
    for val in list(map(int, sys.stdin.readline().split())):
        cnt[val] += 1
    fac[0] = fac[1] = 1
    finv[0] = finv[1] = 1
    inv[1] = 1
    for i in range(2, n):
        inv[i] = MOD - inv[MOD % i] * (MOD // i) % MOD
        fac[i] = fac[i - 1] * i % MOD
        finv[i] = finv[i - 1] * inv[i] % MOD
    id, ad, ans, s, t, pl = 0, 0, B, 0, 0, B
    flag = False
    for i in range(n-1, -1, -1):
        if cnt[i] == 0:
            continue
        s = prod(id + cnt[i] - 1, cnt[i] - 1) * cnt[i]
        t = (s * B + prod(id + cnt[i] - 1, cnt[i])) % MOD
        if t > 0:
            ad += s * mod_inverse(t)
            pl = pl * t % MOD
        else:
            if flag:
                pl = 0
            else:
                flag = True
                pl = pl * s % MOD
        ans = ans * t % MOD
        id += cnt[i]
    print((ans * ad + (pl if flag else 0)) % MOD)