#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

/* msi1427 */

#define ll           long long
#define ull          long long unsigned
#define LD           long double
#define FOR(i,n)     for(int i=0;i<n;i++)
#define FLR(i,n)     for(ll i=0;i<n;i++)
#define RFOR(i,n)    for(int i=n-1;i>=0;i--)
#define RFLR(i,n)    for(ll i=n-1;i>=0;i--)
#define rep(i,a,b)   for(int i=a;i<=b;i++)
#define repl(i,a,b)  for(ll i=a;i<=b;i++)
#define casep(z)     cout << "Case " << z << ": ";
#define Fast         ios_base::sync_with_stdio(0);cin.tie(0);
#define sz(a)        a.size()
#define all(x)       x.begin(),x.end()
#define SORT(a,n)    sort(a,a+n)
#define RSORT(a,n)   sort(a,a+n,greater<ll>())
#define pii          pair<int,int>
#define pll          pair<ll,ll>
#define tll          tuple<ll,ll,ll>
#define VSORT(v)     sort(all(v))
#define VRSORT(v)    sort(v.rbegin(),v.rend())
#define pb           push_back
#define eb           emplace_back
#define pf           push_front
#define pob          pop_back()
#define pof          pop_front()
#define ff           first
#define ss           second
#define tt           true
#define fs           false
#define MP           make_pair
#define MEM(x,n)     memset((x),n,sizeof(x))
#define bsrch(a,n,x) binary_search(a,a+n,x)
#define vi           vector<int>
#define vll          vector<ll>
#define PI           acos(-1.0)
#define LB(a,x)      (lower_bound(all(a),x)-a.begin()) //  first element in the range [first,last) which does not compare less than val.
#define UB(a,x)      (upper_bound(all(a),x)-a.begin()) //  first element in the range [first,last) which compares greater than val.
#define PQ           priority_queue
#define heap         PQ< ll , vector<ll> , greater < ll > >
#define REV(x)       reverse(all(x))
#define mpii         map<int,int>
#define mpll         map<ll,ll>
#define cig          cin.ignore()
#define INF          (ll)1e18
#define EPS          1e-9
#define prec(n)      fixed << setprecision(n)
#define lb           lower_bound
#define ub           upper_bound
#define fin          freopen("in.txt","r",stdin)
#define fout         freopen("out.txt","w",stdout)
#define sc           scanf
#define pt           printf
#define np           next_permutation
#define IT(it,x)     for(auto it=x.begin();it!=x.end();it++)
#define sci(x)       scanf("%d",&x);
#define scl(x)       scanf("%lld",&x);
#define scd(x)       scanf("%lf",&x);
#define scs(x)       scanf("%s",x);

#define min3(a,b,c)     min(a,min(b,c))
#define max3(a,b,c)     max(a,max(b,c))
#define min4(a,b,c,d)   min(min(a,b),min(c,d))
#define max4(a,b,c,d)   max(max(a,b),max(c,d))
#define max5(a,b,c,d,e) max(max3(a,b,c),max(d,e))
#define min5(a,b,c,d,e) min(min3(a,b,c),min(d,e))
#define EulerGamma      0.57721566490153286060651209

typedef
tree<
  ll,
  null_type,
  less<ll>,
  rb_tree_tag,
  tree_order_statistics_node_update>
ordered_set;

ll dx[] = {1,-1,0,0};
ll dy[] = {0,0,1,-1};

ll ceiling(ll x,ll y)
{
    if(x%y) return ((x/y)+1);
    else return (x/y);
}

ll gcd(ll x,ll y)
{
    return ((y==0)?x:gcd(y,x%y));
}

ll lcm(ll x,ll y)
{
    return ((x*y)/gcd(x,y));
}

ll BMod(ll B,ll P,ll M)
{
    ll R=1;
    B=B%M;
    while(P)
    {
        if(P&1)
            R=(R*B)%M;
        P >>= 1;
        B=(B*B)%M;
    }
    return R;
}

bool check(ll n,ll pos) {return (n&(1<<pos));}
ll SET(ll n,ll pos){return (n|(1<<pos));}
ll RESET(ll n,ll pos) {return(n&~(1<<pos));}
mt19937 rng32(chrono::steady_clock::now().time_since_epoch().count());

#define MOD 1000000007
#define MAX 1000005

ll n,k,fact[MAX];

ll nCr(ll n, ll r){
    ll up = fact[n]%MOD;
    ll down = (fact[r]*fact[n-r])%MOD;
    ll out = (up*BMod(down,MOD-2,MOD))%MOD;
    return out;
}

int main()
{
    Fast
    fact[0]=1;
    repl(i,1,MAX-1) fact[i]=(fact[i-1]*i)%MOD;
    cin>>n>>k;
    vector<ll>v(n+1);
    repl(i,1,n) {
        cin>>v[i];
    }
    if(k==1){cout << 0 << "\n";return 0;}
    sort(v.begin()+1,v.begin()+n+1);
    ll cnt = 0;
    repl(i,1,n){
        if(i-1>=k-1){
            cnt+=nCr(i-1,k-1)*v[i];
            cnt%=MOD;
        }
        if(n-i>=k-1){
            cnt-=nCr(n-i,k-1)*v[i];
            cnt=(cnt+MOD)%MOD;
        }
    }
    cout << cnt%MOD << "\n";
}
/*
6 6
1 2 6 8 10 14
*/


Measured memory_bytes: 8832