/*
TASK: seq
LANG: C++
*/

#include <cstdio>
#include <iostream>
using namespace std;

long long n,k,sum;

void read()
{
	long long x;
	cin>>n>>k;
	for(long long i=0;i<n;i++)
	{
		cin>>x;
		sum+=x;
	}
	if(k==0)
	{
		cout<<sum<<endl;
		exit(0);
	}
	long long pr=1;
	for(long long i=n-k+1;i<=n;i++) pr*=i;
	for(long long i=1;i<=k;i++) pr/=i;
	if(k==1)
	{
		cout<<(pr-1)*sum<<endl;
		exit(0);
	}
	n--;
	k--;
	long long pm=1;
	for(long long i=n-k+1;i<=n;i++) pm*=i;
	for(long long i=1;i<=k;i++) pm/=i;
	cout<<(pr-pm)*sum<<endl;
}

int main()
{
	read();
    return 0;
}
