#include<bits/stdc++.h>
using namespace std;
const int N=1e5+50,mod=1e9+7;
int n,k,m,a[N],c[N],b[N],J[N],I[N],ans;
int C(int n,int m){return n<m?0:1ll*J[n]*I[m]%mod*I[n-m]%mod;}
int main(){
    // freopen("test.in","r",stdin);
    scanf("%d%d",&n,&k);J[0]=I[0]=I[1]=1;
    for(int i=2;i<=n;i++)I[i]=mod-1ll*mod/i*I[mod%i]%mod;
    for(int i=1;i<=n;i++)J[i]=1ll*J[i-1]*i%mod,I[i]=1ll*I[i-1]*I[i]%mod;
    for(int i=1;i<=n;i++)scanf("%d",&a[i]),c[i]=a[i];
    sort(c+1,c+n+1);m=unique(c+1,c+n+1)-c-1;
    for(int i=1;i<=n;i++){
        a[i]=lower_bound(c+1,c+m+1,a[i])-c;
        b[a[i]]++;
    }
    for(int i=1,sum=0;i<=m;i++){
        ans=(ans+1ll*(C(sum+b[i],k)-C(sum,k))*c[i])%mod;
        sum+=b[i];
    }
    for(int i=m,sum=0;i;i--){
        ans=(ans-1ll*(C(sum+b[i],k)-C(sum,k))*c[i])%mod;
        sum+=b[i];
    }
    printf("%d\n",(ans+mod)%mod);
    return 0;
}