#include <bits/stdc++.h>
using namespace std;

#define all(v)                      v.begin(), v.end()
#define Chrono                      chrono::steady_clock::now().time_since_epoch().count()
#define dist2D(x1, y1, x2, y2)            ((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2))
#define dist3D(x1, y1, z1, x2, y2, z2)    ((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2) + (z1 - z2) * (z1 - z2))
#define EPS                         1e-15
#define eb                          emplace_back
#define endl                        "\n"
#define FastIO                      ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define FI                          freopen("in.txt", "r", stdin)
#define FO                          freopen("out.txt", "w", stdout)
#define ff                          first
#define gt(i, x)                    get<i>(x)
#define INF              1000000000000000000
#define ld                          long double
#define ll                          long long
#define mem(x,y)                    memset(x, y, sizeof x)
#define mp                          make_pair
#define mt                          make_tuple
#define msi                         map<string, int>
#define mii                         map<int, int>
#define mis                         map<int, string>
#define MOD                         1000000007
#define PI                          acos(-1.0)
#define PQ                          priority_queue
#define pb                          push_back
#define pib                         pair<int, bool>
#define pii                         pair<int, int>
#define iii                         pair<int, pii>
#define iiii                        pair<pii, pii>
#define pll                         pair<ll, ll>
#define present(c, x)               ((c).find(x) != (c).end())
#define sfi(x)                      scanf("%d", &x)
#define sfii(x,y)                   scanf("%d %d", &x, &y)
#define sfiii(x,y,z)                scanf("%d %d %d", &x, &y, &z)
#define siz(x)                      (int)x.size()
#define ss                          second
#define tii                         tuple<int, int>
#define tiii                        tuple<int, int, int>
#define tiiii                       tuple<int, int, int, int>
#define timeTaken                   endd = clock(); cerr << (double) (endd - beginn) / CLOCKS_PER_SEC * 1000 << endl
#define timeInit                    clock_t beginn = clock(), endd
#define ull                         unsigned long long
#define umsi                        unordered_map<string, int>
#define umii                        unordered_map<int, int>
#define umis                        unordered_map<int, string>
#define vb                          vector<bool>
#define vi                          vector<int>
#define vvi                         vector<vi>
#define vii                         vector<pii>
#define vvii                        vector<vii>
#define vll                         vector<ll>
#define vvll                        vector<vll>
#define vpll                        vector<pll>
#define bug(...)                    cerr << __LINE__ << " : (" << #__VA_ARGS__ << ") = ("; _Print(__VA_ARGS__);
template<class T> void _Print(T &&x) { cerr << x << ")" << endl; }
template<class T, class ...S> void _Print(T &&x, S &&...y) { cerr << x << ", "; _Print(y...); }


#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <functional>
using namespace __gnu_pbds;
typedef tree<ll,null_type, less<ll>, rb_tree_tag,
        tree_order_statistics_node_update> ordered_set;

//os.order_of_key(v): returns how many elements strictly less than v
//os.find_by_order(k - 1): returns kth smallest element's iterator

template<class T> class compare {
public:
    bool operator() (pair<T, T> &x, pair<T, T> &y) {
        if (x.first == y.first) {
            return x.ss > y.ss;
        }
        return x.ff > y.ff;
    }
};

template<class T> ostream &operator<<(ostream &os, const pair<T, T> &a) { return os << a.ff << " " << a.ss; }
ll bigmod(ll a, ll b) {
    ll po = 1;
    while (b) {
        if (b & 1)
            po = po * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return po;
}

template<class T> pair<T, T> operator+(const pair<T, T> &a, const pair<T, T> &b) { return {a.ff + b.ff, a.ss + b.ss}; }
template<class T> pair<T, T> operator-(const pair<T, T> &a, const pair<T, T> &b) { return {a.ff - b.ff, a.ss - b.ss}; }
template<class T> pair<T, T> operator*(const pair<T, T> &a, const pair<T, T> &b) { return {a.ff * b.ff, a.ss * b.ss}; }
template<class T> pair<T, T> operator%(const pair<T, T> &a, const pair<T, T> &b) { return {a.ff % b.ff, a.ss % b.ss}; }
template<class T, class U> pair<T, T> operator+(const pair<T, T> &a, const U &b) { return {a.ff + b, a.ss + b}; }
template<class T, class U> pair<T, T> operator*(const pair<T, T> &a, const U &b) { return {a.ff * b, a.ss * b}; }

int Set(int N, int pos) { return N = N | (1 << pos); }
int reset(int N, int pos){ return N = N & ~ (1 << pos); }
bool check(int N, int pos){ return (bool) (N & (1 << pos)); }

///=======================================template=======================================///

ll fact[100005], factinv[100005];
ll nCr(ll n, ll r) {
    return fact[n] * factinv[n - r] % MOD * factinv[r] % MOD;
}

int main() {
    //FI;
    //FO;
    FastIO; //timeInit;
    fact[0] = 1;
    for (int i = 1; i <= 100000; i++) {
        fact[i] = fact[i - 1] * i % MOD;
    }

    factinv[100000] = bigmod(fact[100000], MOD - 2) % MOD;
    for (int i = 99999; i >= 0; i--) {
        factinv[i] = factinv[i + 1] * (i + 1) % MOD;
    }

    int n, k;
    cin >> n >> k;
    int ara[n];
    for (auto &x : ara)
        cin >> x;
    sort(ara, ara + n);
    ll ans = 0;

    for (int i = k - 1; i < n; i++) {
        ans = (ans + nCr(i, k - 1) * ara[i] % MOD) % MOD;
    }

    for (int i = 0; i <= n - k; i++) {
        ans = (ans - nCr(n - i - 1, k - 1) * ara[i] % MOD + MOD) % MOD;
    }

    cout << ans << endl;

    //timeTaken;
    return 0;
}

Measured memory_bytes: 2176