#include #ifdef DEBUG #include #else #define dump(...) #endif /** * @title Modint * @docs mint.md */ template class ModInt{ public: constexpr static uint32_t MOD = M; uint64_t val; constexpr ModInt(): val(0){} constexpr ModInt(int64_t n){ if(n >= M) val = n % M; else if(n < 0) val = n % M + M; else val = n; } inline constexpr auto operator+(const ModInt &a) const {return ModInt(val + a.val);} inline constexpr auto operator-(const ModInt &a) const {return ModInt(val - a.val);} inline constexpr auto operator*(const ModInt &a) const {return ModInt(val * a.val);} inline constexpr auto operator/(const ModInt &a) const {return ModInt(val * a.inv().val);} inline constexpr auto& operator=(const ModInt &a){val = a.val; return *this;} inline constexpr auto& operator+=(const ModInt &a){if((val += a.val) >= M) val -= M; return *this;} inline constexpr auto& operator-=(const ModInt &a){if(val < a.val) val += M; val -= a.val; return *this;} inline constexpr auto& operator*=(const ModInt &a){(val *= a.val) %= M; return *this;} inline constexpr auto& operator/=(const ModInt &a){(val *= a.inv().val) %= M; return *this;} inline constexpr bool operator==(const ModInt &a) const {return val == a.val;} inline constexpr bool operator!=(const ModInt &a) const {return val != a.val;} inline constexpr auto& operator++(){*this += 1; return *this;} inline constexpr auto& operator--(){*this -= 1; return *this;} inline constexpr auto operator++(int){auto t = *this; *this += 1; return t;} inline constexpr auto operator--(int){auto t = *this; *this -= 1; return t;} inline constexpr static ModInt power(int64_t n, int64_t p){ if(p < 0) return power(n, -p).inv(); int64_t ret = 1, e = n % M; for(; p; (e *= e) %= M, p >>= 1) if(p & 1) (ret *= e) %= M; return ret; } inline constexpr static ModInt inv(int64_t a){ int64_t b = M, u = 1, v = 0; while(b){ int64_t t = a / b; a -= t * b; std::swap(a,b); u -= t * v; std::swap(u,v); } u %= M; if(u < 0) u += M; return u; } inline constexpr static auto frac(int64_t a, int64_t b){return ModInt(a) / ModInt(b);} inline constexpr auto power(int64_t p) const {return power(val, p);} inline constexpr auto inv() const {return inv(val);} friend inline constexpr auto operator-(const ModInt &a){return ModInt(-a.val);} friend inline constexpr auto operator+(int64_t a, const ModInt &b){return ModInt(a) + b;} friend inline constexpr auto operator-(int64_t a, const ModInt &b){return ModInt(a) - b;} friend inline constexpr auto operator*(int64_t a, const ModInt &b){return ModInt(a) * b;} friend inline constexpr auto operator/(int64_t a, const ModInt &b){return ModInt(a) / b;} friend std::istream& operator>>(std::istream &s, ModInt &a){s >> a.val; return s;} friend std::ostream& operator<<(std::ostream &s, const ModInt &a){s << a.val; return s;} template inline static auto div(){ static auto value = inv(N); return value; } explicit operator int32_t() const noexcept {return val;} explicit operator int64_t() const noexcept {return val;} }; using mint = ModInt<1000000007>; template struct BigCombination{ T kfinv; std::vector f, finv; int l; BigCombination(int K, int l, int r): l(l){ const int len = r - l + 1; kfinv = 1; for(int i = 1; i <= K; ++i) kfinv *= i; kfinv = kfinv.inv(); f.resize(len); finv.resize(len); f[0] = 1; for(int i = 1; i <= l; ++i) f[0] *= i; for(int i = l + 1; i <= r; ++i) f[i - l] = f[i - l - 1] * i; finv.back() = 1; for(int i = 1; i <= r - K; ++i) finv.back() *= i; finv.back() = finv.back().inv(); for(int i = r - K - 1; i >= l - K; --i) finv[i - (l - K)] = finv[i - (l - K) + 1] * (i + 1); } T operator[](int n) const {return kfinv * f[n - l] * finv[n - l];} }; int main(){ std::cin.tie(0); std::ios::sync_with_stdio(false); int N, K; while(std::cin >> N >> K){ std::vector A(N); for(int i = 0; i < N; ++i) std::cin >> A[i]; BigCombination bc(K, K, K + N); mint ans = 0; for(int i = 0; i < N; ++i){ ans += bc[K + i] * bc[K + N - i - 1] * A[i]; } std::cout << ans << "\n"; } return 0; }