#include <bits/stdc++.h>
#define endl '\n'
#define int long long
#define lint long long
#define pii pair<int, int>
#define FOR(i, a, b) for(int i = (a); i < (b); ++i)
#define FORR(i, a, b) for(int i = (a); i > (b); --i)
#define REP(i, n) FOR(i, 0, n)
#define SZ(v) ((int)v.size())
#define ZERO(a) memset(a, 0, sizeof(a))
#define MINUS(a) memset(a, 0xff, sizeof(a))
#define FILLINF(a) memset(a, 0x3f, sizeof(a))
#define POW(n) (1LL << (n))
#define POPCNT(n) (__builtin_popcount(n))
#define IN(i, a, b) ((a) <= (i) && (i) <= (b))
using namespace std;
template <typename T> inline bool CHMIN(T& a,T b) { if(a > b) { a = b; return 1; } return 0; }
template <typename T> inline bool CHMAX(T& a,T b) { if(a < b) { a = b; return 1; } return 0; }
template <typename T> inline void SORT(T& a) { sort(begin(a), end(a)); }
template <typename T> inline void REV(T& a) { reverse(begin(a), end(a)); }
template <typename T> inline void UNI(T& a) { sort(a.begin(), a.end()); a.erase(unique(a.begin(), a.end()), a.end()); }
template <typename S, typename T> inline T LB(S& v, T a) { return *lower_bound(begin(v), end(v), a); }
template <typename S, typename T> inline int LBP(S& v, T a) { return lower_bound(begin(v), end(v), a) - begin(v); }
template <typename S, typename T> inline T UB(S& v, T a) { return *upper_bound(begin(v), end(v), a); }
template <typename S, typename T> inline int UBP(S& v, T a) { return upper_bound(begin(v), end(v), a) - begin(v); }
template <typename S, typename T> ostream& operator << (ostream& os, const pair<S, T>& p) { os << p.first << " " << p.second; return os; }
template <typename S, typename T> istream& operator >> (istream& is, pair<S, T>& p) { is >> p.first >> p.second; return is; }
template <typename T> ostream& operator << (ostream& os, const vector<T>& v) { for (int i = 0; i < v.size(); ++i) { if (i) os << " "; os << v[i]; } return os; }
template <typename T> istream& operator >> (istream& is, vector<T>& v) { for(T& in : v) is >> in; return is; }
template <typename T = int> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...)); }
template <typename T, typename V> typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) { t = v; }
template <typename T, typename V> typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) { for(auto &e : t) fill_v(e, v); }
const double PI = 3.14159265358979323846;
const double EPS = 1e-10;
const lint INF = 0x3f3f3f3f3f3f3f3f;
const lint MOD = 1000000007;

template <int MOD> class Combination {
    vector<long long> inv, fac, finv;

    void init(int n) {
        inv[1] = 1;
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        for (int i = 2; i <= n; ++i) {
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
            fac[i] = fac[i - 1] * i % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

public:
    Combination(int n) : inv(n + 1), fac(n + 1), finv(n + 1) { init(n); }

    long long P(int n, int k) {
        if (n < k || n < 0 || k < 0) return 0;
        return fac[n] * finv[n - k] % MOD;
    }

    long long C(int n, int k) {
        if (n < k || n < 0 || k < 0) return 0;
        return fac[n] * finv[k] % MOD * finv[n - k] % MOD;
    }

    long long H(int n, int k) {
        if (n < 0 || k < 0) return 0;
        return C(n + k - 1, k);
    }

    long long Fac(int n) {
        return fac[n];
    }

    long long Inv(int n) {
        return inv[n];
    }

    long long Finv(int n) {
        return finv[n];
    }
};

int N, K;
int A[100010];
int cum[100010];

void _main() {
    cin >> N >> K;
    REP (i, N) cin >> A[i];
    sort(A, A + N);
    if (K == 1) {
        cout << 0 << endl;
        return;
    }
    Combination<MOD> C(100010);
    FOR (i, K - 2, N) {
        cum[i] = C.C(i, K - 2);
        if (i) (cum[i] += cum[i - 1]) %= MOD;
    }
    int ans = 0;
    REP (i, N) {
        if (i - 1 >= K - 2) (ans += cum[i - 1] * A[i] % MOD) %= MOD;
        if (N - i - 2 >= K - 2) (ans += MOD - cum[N - i - 2] * A[i] % MOD) %= MOD;
    }
    cout << ans << endl;
}

signed main(signed argc, char **argv) {
    if (argc > 1) {
        if (strchr(argv[1], 'i')) freopen("input.txt", "r", stdin);
        if (strchr(argv[1], 'o')) freopen("output.txt", "w", stdout);
    }
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(10);
    _main();
    return 0;
}

Measured memory_bytes: 3840