# include <bits/stdc++.h>
# define rep(i, n) for(int i=0, i##_len=(n); i<i##_len; ++i)
# define reps(i, n) for(int i=1, i##_len=(n); i<=i##_len; ++i)
# define rrep(i, n) for(int i=((int)(n)-1); i>=0; --i)
# define rreps(i, n) for(int i=((int)(n)); i>0; --i)
# define range_for(i, b, e) for(int i=(b), i##_len=(e); i<i##_len; ++i)
# define step(n) rep(_, n)
# define ALL(x) (x).begin(), (x).end()
# define RALL(x) (x).rbegin(), (x).rend()
# define Unique(a) sort((a).begin(), (a).end()); (a).erase(unique((a).begin(), (a).end()), (a).end())
# define pb push_back
# define len(x) ((int)(x).size())
# define optimize_cin() cin.tie(0); ios::sync_with_stdio(false)
# define debug(x) std::cerr<<#x<<": "<<(x)<<endl;
# define LINT_MAX (LLONG_MAX)
# define LINT_MIN (LLONG_MIN)
# define cauto const auto
# define int long long
# define float long double
using namespace std;
template <class Type> inline constexpr Type Square(Type x) { return x * x; }
template <class Type> inline constexpr bool InRange(const Type& x, const Type& fst, const Type& lst) { return (fst <= x) && (x < lst); }
template<class Integer> inline bool chmax(Integer &a, Integer b) { return a < b && (a = b, true); }
template<class Integer> inline bool chmin(Integer &a, Integer b) { return a > b && (a = b, true); }
template<class Integer>bool IsOdd(Integer &n) { return n & 1; }
template<class Integer>bool IsEven(Integer &n) { return !(n & 1); }
long long gcd(long long a, long long b) { while(b){ long long A = a; (a = b), (b = A % b); } return a; }
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
int ctoi(const char c) { return ('0' <= c && c <= '9') ? (c - '0') : -1; }
string YesNo(bool b) { return b ? "Yes" : "No"; }
string YESNO(bool b) { return b ? "YES" : "NO"; }
string yesno(bool b) { return b ? "yes" : "no"; }
void _cin(){} template <class Head, class... Tail> void _cin(Head&& head, Tail&&... tail){ cin >> head; _cin(forward<Tail>(tail)...); }
#define Cin(Type, ...) Type __VA_ARGS__; _cin(__VA_ARGS__)
#define Cinv(Type, xs, n) vector<Type> xs(n); rep(i, n) cin >> xs[i]
#define Cinv2(Type, xs, ys, n) vector<Type> xs(n), ys(n); rep(i, n) cin >> xs[i] >> ys[i]
#define Cinv3(Type, xs, ys, zs, n) vector<Type> xs(n), ys(n), zs(n); rep(i, n) cin >> xs[i] >> ys[i] >> zs[i]
#define Cinvv(Type, xs, h, w) vector<vector<Type>> xs(h, vector<int>(w)); rep(i, h) rep(j, w) cin >> xs[i][j]
void Print() { cout << endl; }
template <class Head, class... Tail> void Print(Head&& head, Tail&&... tail) { cout << head; if (sizeof...(tail) != 0) cout << " "; Print(forward<Tail>(tail)...); }
template <class Type> void Print(vector<Type> &vec) { for (auto& a : vec) { cout << a; if (&a != &vec.back()) cout << " "; } cout << endl; }
template <class Type> void Print(vector<vector<Type>> &df) { for (auto& vec : df) { Print(vec); } }
void Debug() { cerr << endl; }
template <class Head, class... Tail> void Debug(Head&& head, Tail&&... tail) { cerr << head; if (sizeof...(tail) != 0) cerr << " "; Debug(forward<Tail>(tail)...); }
template <class Type> void Debug(vector<Type> &vec) { for (auto& a : vec) { cerr << a; if (&a != &vec.back()) cerr << " "; } cerr << endl; }
template <class Type> void Debug(vector<vector<Type>> &df) { for (auto& vec : df) { Debug(vec); } }

static const int MOD = 1000000007;
struct mint {
    long long x;
    mint(long long x=0):x((x%MOD+MOD)%MOD){}
    mint operator-() const { return mint(-x);}
    mint& operator+=(const mint a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator-=(const mint a) {
        if ((x += MOD-a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator*=(const mint a) { (x *= a.x) %= MOD; return *this;}
    mint operator+(const mint a) const { return mint(*this) += a;}
    mint operator-(const mint a) const { return mint(*this) -= a;}
    mint operator*(const mint a) const { return mint(*this) *= a;}
    mint pow(long long t) const {
    if (!t) return 1;
        mint a = pow(t>>1);
        a *= a;
        if (t&1) a *= *this;
        return a;
    }

    // for prime MOD
    mint inv() const { return pow(MOD-2); }
    mint& operator/=(const mint a) { return *this *= a.inv();}
    mint operator/(const mint a) const { return mint(*this) /= a;}
    bool operator<(const mint& iValue) const { return this->x < iValue.x; }
    bool operator<=(const mint& iValue) const { return this->x <= iValue.x; }
    bool operator>(const mint& iValue) const { return this->x > iValue.x; }
    bool operator>=(const mint& iValue) const { return this->x >= iValue.x; }
};
inline std::istream& operator >>(std::istream& is, const mint& a) { return is >> a.x; }
inline std::ostream& operator<<(std::ostream& os, const mint& a) { return os << a.x; }


static const int MAX = 1e5 + 10;

int fac[MAX], finv[MAX], inv[MAX];
 
void COMinit() // 二項係数を求める時は前処理としてCOMinit()をする
{
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++)
    {
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}
 
// 二項係数計算
int COM(int n, int k)
{
    if (n < k)
        return 0;
    if (n < 0 || k < 0)
        return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

signed main()
{
    COMinit();
    Cin(int, N, K);
    Cinv(int, A, N);

    sort(ALL(A));

    int ans = 0;
    rep (i, N)
    {
        ans = (ans - (COM(N-i-1, K-1)*A[i])%MOD)%MOD;
        ans = (ans + (COM(i, K-1)*A[i])%MOD)%MOD;
    }

    Print(ans);

    return 0;
}


Measured memory_bytes: 6392