public static void Main(string[] args)
{
int N = ri;
int K = ri;
int[] A = ria;
Mint ans = 0;
Sort(A);
if (K == 1)
{
Console.WriteLine(0);
return;
}
Mint[] maxCounts = new Mint[N];
Mint[] minCounts = new Mint[N];
for (int i = 0; i < N; i++)
{
// A[i]が最大値になる回数
maxCounts[i] = Combination.nCk(i, K - 1);
// A[i]が最小値になる回数
minCounts[i] = Combination.nCk(N - i - 1, K - 1);
}
for (int i = 0; i < N; i++)
{
ans += A[i] * maxCounts[i];
ans -= A[i] * minCounts[i];
}
Console.WriteLine(ans);
}
public static Mint nCkForBigNSmallK(long n, long k)
{
Mint x = 1; Mint y = 1; for (int i = 0; i < k; i++)
{
x *= n - i; y *= i + 1;
}
return(x / y);
}
} } public class Combination { static long SIZE = (long)1e7 + 1; static Mint[] mFactorial, mFactrialInverse, mInverse; private static void Init()
{
mFactorial = new Mint[SIZE]; mFactrialInverse = new Mint[SIZE]; mInverse = new Mint[SIZE]; mFactorial[0] = mFactorial[1] = 1; mFactrialInverse[0] = mFactrialInverse[1] = 1; mInverse[1] = 1; for (int i = 2; i < SIZE; i++)
{
mFactorial[i] = mFactorial[i - 1] * i; mInverse[i] = -1 * mInverse[Mint.MOD % i] * (Mint.MOD / i); mFactrialInverse[i] = mFactrialInverse[i - 1] * mInverse[i];
}
}