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


//TEMPLATE
#define pb                push_back
#define mp                make_pair
#define ll                long long
#define ld                long double
#define pii               pair <ll, ll>
#define piii              pair <ll, pii>
#define F                 first
#define S                 second

#define newline           printf("\n")
#define minusone          printf("-1\n")
#define zeroo             printf("0\n")
#define scl1(a)           scanf("%lld", &a)
#define scl2(a, b)        scanf("%lld %lld", &a, &b)
#define scl3(a, b, c)     scanf("%lld %lld %lld", &a, &b, &c)
#define prl1(a)           printf("%lld\n", a)
#define prl2(a, b)        printf("%lld %lld\n", a, b)
#define ssl1(s)           scanf(" %[^\n]", s)
#define scd1(a)           scanf("%lf", &a)
#define scd2(a, b)        scanf("%lf %lf", &a, &b)
#define prd(a)            printf("%lf\n", a)
#define prld(a)           printf("%Lf\n", a)
#define prcase(cs)        printf("Case %lld: ", cs)
#define cin1(a)           cin >> a
#define cin2(a, b)        cin >> a >> b
#define cin3(a, b, c)     cin >> a >> b >> c
#define cin4(a, b, c, d)  cin >> a >> b >> c >> d
#define cot1(a)           cout << a << "\n"
#define cot2(a, b)        cout << a << " " << b << "\n"
#define cot3(a, b, c)     cout << a << " " << b << " " << c << "\n"
#define cot4(a, b, c, d)  cout << a << " " << b << " " << c << " " << d << "\n"
#define cotline           cout << "\n"
#define cotminus          cout << "-1\n"
#define cot0              cout << "0\n"
#define cotyes            cout << "YES\n"
#define cotno             cout << "NO\n"
#define cotcase(cs)       cout << "Case " << cs << ": "

#define reversed(s)       reverse(s.begin(), s.end())
#define asort(s)          sort(s.begin(), s.end())
#define dsort(s)          sort(s.rbegin(), s.rend())
#define all(s)            s.begin(), s.end()
#define uniq(s)           s.resize(distance(s.begin(),unique(s.begin(), s.end())))
#define found(s, x)       (s.find(x) != s.end())

#define for0(i, n)        for (i = 0; i < n; i++)
#define for1(i, n)        for (i = 1; i <= n; i++)
#define fora(i, a, b)     for (i = a; i <= b; i++)
#define forb(i, b, a)     for (i = b; i >= a; i--)
#define fori(it, s)       for(auto it = s.begin(); it != s.end(); it++)

#define FR                ios_base::sync_with_stdio(false);cin.tie(NULL)
#define ms(a, x)          memset(a, x, sizeof a)
#define bitcount(n)       __builtin_popcountll(n)

//#define pi              3.1415926536
#define pi                acos(-1)

const ll INF = LLONG_MAX;
const ll SZ = 1e5+5;
const ll mod = 1e9+7;


ll c1, c2;
ll ex_euclid(ll a, ll b) {
    if(b == 0) {
        c1 = 1;
        c2 = 0;
        return a;
    }

    ll g = ex_euclid(b, a%b);

    ll x1 = c1;
    c1 = c2;
    c2 = x1 - c2 * (a/b);

    return g;
}

ll mod_inverse(ll a, ll m) {
    ll g = ex_euclid(a, m);

    return ((c1%m) + m) % m;
}

ll ft[SZ], invf[SZ];
void cal_fact() {
    ll i;
    ft[0] = 1;
    invf[0] = mod_inverse(ft[0], mod);

    for1(i, SZ-2) {
        ft[i] = (ft[i-1] * i) % mod;
        invf[i] = mod_inverse(ft[i], mod);
    }
}

ll ncr_mod(ll x, ll y) {
    if(x < y) return 0;
    ll r1 = (ft[x] * invf[y]) % mod;
    ll res = (r1 * invf[x-y]) % mod;
    return res;
}

ll n;
ll a[SZ];


int main() {
    cal_fact();
    ll cs = 0, tc = 1, x, y, z, i, j, k, g, p, q, ans = 0, sum = 0, c = 0, t = 0;
//    ll a, b, d;
    string s, s1, s2;

    cin2(n, k);
    for1(i, n) cin1(a[i]);

    sort(a+1, a+n+1);

    ll mn(0), mx(0);

    for1(i, n) {
        c = ncr_mod(i-1, k-1);
        mx = (mx + (c * a[i])%mod) % mod;

        c = ncr_mod(n-i, k-1);
        mn = (mn + (c * a[i])%mod) % mod;
    }

//    cot2(mx, mn);

    ans = (mx - mn + mod) % mod;
    cot1(ans);

    return 0;
}



Measured memory_bytes: 5952