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


#define li          long long int
#define rep(i,to)   for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)    for(li i=(li)(start);i<((li)(to));i++)
#define pb          push_back
#define sz(v)       ((li)(v).size())
#define bgn(v)      ((v).begin())
#define eend(v)     ((v).end())
#define allof(v)    (v).begin(), (v).end()
#define dodp(v,n)       memset(v,(li)n,sizeof(v))
#define bit(n)      (1ll<<(li)(n))
#define mp(a,b)     make_pair(a,b)
#define rin rep(i,n)
#define EPS 1e-12
#define ETOL 1e-8
#define MOD 1000000007
typedef pair<li, li> PI;

#define INF bit(60)

#define DBGP 1


#define idp if(DBGP)
#define F first
#define S second
#define p2(a,b)     idp cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)       idp cout<<a<<"\t"<<b<<"\t"<<c<<endl
#define p4(a,b,c,d)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<endl
#define p5(a,b,c,d,e)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<endl
#define p6(a,b,c,d,e,f)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<endl
#define p7(a,b,c,d,e,f,g)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<endl
#define p8(a,b,c,d,e,f,g,h)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<endl
#define p9(a,b,c,d,e,f,g,h,i)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<endl
#define p10(a,b,c,d,e,f,g,h,i,j)        idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<"\t"<<j<<endl
#define foreach(it,v)   for(__typeof((v).begin()) it=(v).begin(); it!=(v).end(); ++it)
#define p2p(x)      idp p2((x).F, (x).S)
#define dump(x,n)   idp{rep(i,n){cout<<x[i]<<" ";}puts("");}
#define dump2(x,n)  idp{rep(i,n){cout<<"["<<x[i].F<<" , "<<x[i].S<<"] ";}puts("");}
#define dumpi(x)    idp{foreach(it, x){cout<<(*it)<<" ";}puts("");}
#define dumpi2(x)   idp{foreach(it, x){cout<<"["<<(it)->F<<" , "<<(it)->S<<"] ";}puts("");}

#define read2d(a,w,h)   rep(i,h)rep(j,w)cin>>a[i][j]
#define dump2d(a,w,h)   rep(i,h){rep(j,w)cout<<a[i][j]<<" ";puts("");}

typedef pair<li, li> PI;

li fact_mod[100100];

inline void init() {
    fact_mod[0] = 1;
    fact_mod[1] = 1;
    repp(i, 2, 100100) {
        fact_mod[i] = (fact_mod[i - 1] * i) % MOD;
    }
}

inline li gcd(li a, li b) {
    return b != 0 ? gcd(b, a % b) : a;
}
// a x + b y = gcd(a, b)
inline li extgcd(li a, li b, li &x, li &y) {
    li g = a; x = 1; y = 0;
    if (b != 0) g = extgcd(b, a % b, y, x), y -= (a / b) * x;
    return g;
}

inline li invMod(li a) {
    li x, y;
    if (extgcd(a, MOD, x, y) == 1) return (x + MOD) % MOD;
    else                         return 0; // unsolvable
}

inline li combi(li n, li k) {
    if (k < 0)return 0;
    if (n < k)return 0;
    return (((fact_mod[n] * invMod(fact_mod[k])) % MOD) * invMod(fact_mod[n - k])) % MOD;
}

li n, k;
li a[100100];

int main() {
    init();
    cin >> n >> k;
    rin{
        cin >> a[i];
    }
    map<li, li> cnt;
    rin{
        cnt[a[i]]++;
    }

    li sum_min = 0;
    li rest = n;
    for (auto && kv : cnt) {
        li min_num = (combi(rest, k) - combi(rest - kv.S, k) + MOD) % MOD;
        sum_min = (sum_min + min_num * kv.F % MOD + MOD) % MOD;
        rest -= kv.S;
    }

    li sum_max = 0;
    rest = n;
    for (auto it = cnt.rbegin(); it != cnt.rend(); ++it) {
        li max_num = (combi(rest, k) - combi(rest - it->S, k) + MOD) % MOD;
        sum_max = (sum_max + max_num * it->F % MOD + MOD) % MOD;
        rest -= it->S;
    }

    cout << (sum_max - sum_min + MOD) % MOD << endl;

    return 0;
}

Measured memory_bytes: 10968