lib C fun strtoll(s : UInt8*, p : UInt8**, b : Int32) : Int64 end class String def to_i64 C.strtoll(self, nil, 10) end end struct Mint @@MOD = 1_000_000_007i64 def self.mod @@MOD end def self.zero Mint.new end def self.mod=(mod) @@MOD = mod end @value : Int64 def initialize @value = 0i64 end def initialize(value) @value = value.to_i64 % @@MOD end def initialize(m : Mint) @value = m.value end protected def value=(value : Int64) @value = value end getter value : Int64 def + : self self end def - : self Mint.new(value != 0 ? @@MOD - @value : 0) end def +(m) Mint.new(@value + m.to_i64 % @@MOD) end def +(m : Mint) result = Mint.new result.value = @value + m.value result.value -= @@MOD if result.value > @@MOD result end def -(m) Mint.new(@value - m.to_i64 % @@MOD) end def -(m : Mint) result = Mint.new result.value = @value - m.value result.value += @@MOD if result.value < 0 result end def *(m) Mint.new(@value * m.to_i64) 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 Mint.new(@value * u) end def //(m) self / m end def **(m : Int) t, res = self, Mint.new(1) while m > 0 res *= t if m.odd? t *= t m >>= 1 end res end def ==(m) @value == m.to_i64 end def !=(m) @value != m.to_i64 end def succ self + 1 end def pred self - 1 end def to_i64 : Int64 @value end delegate to_s, to: @value delegate inspect, to: @value end struct Int def to_mint : Mint Mint.new(self) end end class String def to_mint : Mint Mint.new(self) end end n, k = read_line.split.map(&.to_i) a = read_line.split.map(&.to_i) Mint.mod = a.sum.to_i64 puts a.each_with_index.sum { |a, i| a.to_mint * k.to_mint ** (n - 1 - i) }