n,k=map(int, input().split())
an=[int(i) for i in input().split()]
def cmb(n, r, p):
    if (r < 0) or (n < r):
        return 0
    r = min(r, n - r)
    return fact[n] * factinv[r] * factinv[n-r] % p
p = 10 ** 9 + 7
N = 10 ** 5
fact = [1, 1]
factinv = [1, 1]
inv = [0, 1]

for i in range(2, N + 1):
    fact.append((fact[-1] * i) % p)
    inv.append((-inv[p % i] * (p // i)) % p)
    factinv.append((factinv[-1] * inv[-1]) % p)

an=sorted(an)
r_an=list(reversed(an))

max1=0
min1=0
for i in range(1,n-k+2):
    c=cmb(n-i,k-1,mod)
    max1=max1 + an[-i]*c%p
    min1=min1 + r_an[-i]*c%p
print((max1-min1)%p)