#include<cstdio>
#include<algorithm>
#define mod 1000000007
int n,m,f[100005];
long long fac[100005],inv[100005],ans;
long long c(int x,int y){return y<=x?fac[x]*inv[y]%mod*inv[x-y]%mod:0;}
long long fp(long long x,long long y){return (y%2?x:1ll)*(y/2?fp(x*x%mod,y/2):1ll)%mod;}
void pre(int n)
{
	fac[0]=inv[0]=1;
	for (int i=1;i<=n;i++) fac[i]=fac[i-1]*i%mod;
	for (int i=1;i<=n;i++) inv[i]=fp(fac[i],mod-2);
}
int main()
{
	scanf("%d%d",&n,&m);
	pre(std::max(n,m));
	for (int i=1;i<=n;i++) scanf("%d",&f[i]);
	std::sort(f+1,f+n+1);
	for (int i=1;i<=n;i++) ans=(ans+f[i]*c(i-1,m-1))%mod;
	for (int i=1;i<=n;i++) ans=(ans-f[i]*c(n-i,m-1)+mod)%mod;
	printf("%lld\n",ans);
	return 0;
}