#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define rep2(i,m,n) for (int i = m; i < (n); ++i)
#define all(x) (x).begin(),(x).end()
inline int popcount(const int x) { return __builtin_popcount(x);}
template<class T> void chmin(T &a, const T &b) noexcept { if (b < a) a = b;}
template<class T> void chmax(T &a, const T &b) noexcept { if (a < b) a = b;}
template<class T> void drop(const T &x) { std::cout<<x<<endl; exit(0);}
void debug_out() { std::cout << "\n";}
template<class T, class... Args> void debug_out(const T &x, const Args &... args) { std::cout<<x<< " "; debug_out(args...);}
#ifdef _DEBUG
  #define debug(...) debug_out(__VA_ARGS__)
#else
  #define debug(...) 
#endif

struct InitIO{
  InitIO() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << std::fixed << std::setprecision(15);
  }
}init_io;

const int mod = 1000000007;
// const int mod = 998244353;
struct mint {
  ll x;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
  bool operator==(const mint rhs) const { return x == rhs.x; }
  bool operator!=(const mint rhs) const { return x != rhs.x; }
  bool operator<(const mint &a) const{ return x<a.x;};
};
istream& operator>>(istream& is, mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}

template<typename ModInt> struct Combination {
  map<pair<ll,ll>, mint> ncr_mp;
  Combination(){}
  ModInt C(ll n, ll r) {
    if (r < 0 || n < r) return ModInt(0);
    auto key = pair<ll,ll>(n,r);
    if (ncr_mp.count(key)) return ncr_mp[key];
    if (r == 0 or r == n) return ModInt(1);
    if (r == 1 or r == n-1) return ModInt(n);
    //return mp[key] = nCr(n,r-1) * (n-r+1) / r;
    return ncr_mp[key] = C(n-1,r) * n / (n-r);
  }
  ModInt H(ll n, ll r) {
    if (r < 0 || n < 0) return 0;
    return r == 0 ? ModInt(1) : C(n+r-1, r);
  }
  ModInt operator()(ll n, ll k) { return C(n, k);}
};

int main() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n);
  rep(i,n) cin >> a[i];
  mint ans = 0;
  Combination<mint> comb;
  rep(i, n) { //i番目が残るようなlの選び方はi+1個からk個，rの選び方はn-i個からk個取るパターン数と同じ．
    ans += comb.H(i+1, k) * comb.H(n-(i+1)+1, k) * mint(a[i]);
  }
  cout << ans << endl;
  return 0;
}