#include<bits/stdc++.h>
using namespace std;
const int mod=1000000007;

long fac[10000001],inv[10000001],facinv[10000001];

void COMinit(int max)
{
  fac[0]=1;
  fac[1]=1;
  inv[1]=1;
  facinv[0]=1;
  facinv[1]=1;
  for(int i=2;i<=max;i++)
  {
    fac[i]=(fac[i-1]*i)%mod;
    inv[i]=mod-inv[mod%i]*(mod/i)%mod;
    facinv[i]=facinv[i-1]*inv[i]%mod;
  }
}

long COM(long n,long k)
{assert(n>=k);return fac[n]*(facinv[k]*facinv[n-k]%mod)%mod;}

int main()
{
  COMinit(500000);
  long N,M,ans=0;
  cin>>N>>M;
  vector<long> A(N);
  for(int i=0;i<N;i++)
  {cin>>A[i];}
  for(int i=0;i<N;i++)
  {ans=(ans+A[i]*COM(N-i+M-1,M-1)%mod)%mod;}
  cout<<ans<<endl;
}