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#include <cstdio>
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#include <cstring>
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#include <algorithm>
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using namespace std;
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#define LL long long
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const LL P = 1e9 + 7;
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LL F[100005];
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void Init()
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{
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    F[0] = 1;
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    for (int i = 1; i <= 100000; i++)
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        F[i] = i * F[i - 1] % P;
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}
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LL pow_mod(LL a, int b)
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{
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    LL ans = 1;
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    while (b)
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    {
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        if (b & 1)
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            ans = ans * a % P;
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        b >>= 1;
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        a = a * a % P;
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    }
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    return ans;
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}
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LL C(int n, int m)
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{
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    if (m > n || m < 0)
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        return 0;
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    return F[n] * pow_mod(F[m] * F[n - m] % P, P - 2) % P;
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}
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LL a[100005], Has[100005];
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int Sum[100005], n;
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#define lowbit(_) ((_) & (-_))
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void add(int x, int c)
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{
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    for (int i = x; i <= n; i += lowbit(i))
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        Sum[i] += c;
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}
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int Query(int x)
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{
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    int ans = 0;
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    for (int i = x; i > 0; i -= lowbit(i))
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        ans += Sum[i];
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    return ans;
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}
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int fron[100005], nxt[100005];
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int main()
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{
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    Init();
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    int K;
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    scanf("%d%d", &n, &K);
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    for (int i = 1; i <= n; i++)
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    {
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        scanf("%lld", &a[i]);
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    }
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    sort(a + 1, a + n + 1);
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    LL ans = 0;
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    for (int i = 1; i <= n; i++)
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    {
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        ans = (ans + a[i] * C(i - 1, K - 1)) % P;
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    }
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    printf("%lld\n", ans);
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}