struct ModInt @@MOD = 1_000_000_007i64 def self.mod @@MOD end def self.zero ModInt.new(0) end def initialize(n) @n = n.to_i64 % @@MOD end getter n : Int64 def + : self self end def - : self ModInt.new(n != 0 ? @@MOD - @n : 0) end def +(m) ModInt.new(@n + m.to_i64 % @@MOD) end def -(m) ModInt.new(@n - m.to_i64 % @@MOD) end def *(m) ModInt.new(@n * m.to_i64 % @@MOD) end def /(m) raise DivisionByZeroError.new if m == 0 a, b, u, v = m.to_i64, @@MOD, 1i64, 0i64 while b != 0 t = a // b a -= t * b a, b = b, a u -= t * v u, v = v, u end ModInt.new(@n * u) end def //(m) self / m end def **(m) t, res = self, ModInt.new(1) while m > 0 res *= t if m.odd? t *= t m >>= 1 end res end def ==(m) @n == m.to_i64 end def !=(m) @n != m.to_i64 end def succ self + 1 end def pred self - 1 end def to_i64 : Int64 @n end delegate to_s, to: @n delegate inspect, to: @n end n, k = read_line.split.map &.to_i a = read_line.split.map { |i| ModInt.new(i) } comb = Hash(Int32, ModInt).new comb[k] = ModInt.new(1) (k + 1..k + n).each do |i| comb[i] = comb[i - 1] / (i - k) * i end puts a.each_with_index.sum(ModInt.zero) { |v, i| v * comb[i + k]*comb[n - i - 1 + k] }