def solve(io)
  h, w, k = io.get3
  c = io.get_c(h, String)

  a = Array.new_md(h, w, 0)
  c.each_with_index do |ci, i|
    ci.chars.each_with_index do |cij, j|
      a[i][j] = (1 << (cij - 'a'))
    end
  end
  st = Array.new(h) { |i| SegmentTree.new(a[i]) { |a, b| a | b } }

  r = 0
  h.times do |i|
    w.times do |j|
      m = {h - i, w - j}.min
      f1 = (1..m).bsearch do |b|
        st[i, b].map { |sti| sti[j, b] }.reduce { |a, b| a | b }.popcount >= k
      end
      f2 = (1..m).bsearch do |b|
        st[i, b].map { |sti| sti[j, b] }.reduce { |a, b| a | b }.popcount > k
      end
      r += (f2 || (m + 1)) - (f1 || (m + 1))
    end
  end

  io.put r
end

class ProconIO
  def initialize(@ins : IO = STDIN, @outs : IO = STDOUT)
    @buf = IO::Memory.new("")
  end

  def get(k : T.class = Int32) forall T
    get_v(k)
  end

  macro define_get
    {% for i in (2..9) %}
      def get({{ *(1..i).map { |j| "k#{j}".id } }})
        { {{ *(1..i).map { |j| "get(k#{j})".id } }} }
      end
    {% end %}
  end
  define_get

  macro define_getn
    {% for i in (2..9) %}
      def get{{i}}(k : T.class = Int32) forall T
        get({{ *(1..i).map { "k".id } }})
      end
    {% end %}
  end
  define_getn

  def get_a(n : Int, k : T.class = Int32) forall T
    Array.new(n) { get_v(k) }
  end

  def get_c(n : Int, k : T.class = Int32) forall T
    get_a(n, k)
  end

  macro define_get_c
    {% for i in (2..9) %}
      def get_c(n : Int, {{ *(1..i).map { |j| "k#{j}".id } }})
        a = Array.new(n) { get({{ *(1..i).map { |j| "k#{j}".id } }}) }
        { {{ *(1..i).map { |j| "a.map { |e| e[#{j-1}] }".id } }} }
      end
    {% end %}
  end
  define_get_c

  macro define_getn_c
    {% for i in (2..9) %}
      def get{{i}}_c(n : Int, k : T.class = Int32) forall T
        get_c(n, {{ *(1..i).map { "k".id } }})
      end
    {% end %}
  end
  define_getn_c

  def get_m(r : Int, c : Int, k : T.class = Int32) forall T
    Array.new(r) { get_a(c, k) }
  end

  macro define_put
    {% for i in (1..9) %}
      def put({{ *(1..i).map { |j| "v#{j}".id } }}, *, delimiter = " ")
        {% for j in (1..i) %}
          print_v(v{{j}}, delimiter)
          {% if j < i %}@outs << delimiter{% end %}
        {% end %}
        @outs.puts
      end
    {% end %}
  end
  define_put

  def put_e(*vs)
    put(*vs)
    exit
  end

  def put_f(*vs)
    put(*vs)
    @outs.flush
  end


  private def get_v(k : Int32.class); get_token.to_i32; end
  private def get_v(k : Int64.class); get_token.to_i64; end
  private def get_v(k : UInt32.class); get_token.to_u32; end
  private def get_v(k : UInt64.class); get_token.to_u64; end
  private def get_v(k : Float64.class); get_token.to_f64; end
  private def get_v(k : String.class); get_token; end

  private def get_token
    loop do
      token = @buf.gets(' ', chomp: true)
      break token unless token.nil?
      @buf = IO::Memory.new(@ins.read_line)
    end
  end

  private def print_v(v, dlimiter)
    @outs << v
  end

  private def print_v(v : Enumerable, delimiter)
    v.each_with_index do |e, i|
      @outs << e
      @outs << delimiter if i < v.size - 1
    end
  end
end

struct Int
  def cdiv(b : Int)
    (self + b - 1) // b
  end

  def bit?(i : Int)
    bit(i) == 1
  end

  def set_bit(i : Int)
    self | (self.class.new(1) << i)
  end

  def reset_bit(i : Int)
    self & ~(self.class.new(1) << i)
  end

  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "0.34.0") < 0 %}
    def bit_length : Int32
      x = self < 0 ? ~self : self

      if x.is_a?(Int::Primitive)
        Int32.new(sizeof(self) * 8 - x.leading_zeros_count)
      else
        to_s(2).size
      end
    end
  {% end %}
end

struct Float64
  def near_zero?
    self.abs < EPSILON
  end
end

struct Number
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "1.1.0") < 0 %}
    def zero?
      self == 0
    end

    def positive?
      self > 0
    end

    def negative?
      self < 0
    end
  {% end %}

  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "0.36.0") < 0 %}
    def self.additive_identity
      zero
    end

    def self.multiplicative_identity
      new(1)
    end
  {% end %}
end

class Array
  macro new_md(*args, &block)
    {% if !block %}
      {% for arg, i in args[0...-2] %}
        Array.new({{arg}}) {
      {% end %}
      Array.new({{args[-2]}}, {{args[-1]}})
      {% for arg in args[0...-2] %}
        }
      {% end %}
    {% else %}
      {% for arg, i in args %}
        Array.new({{arg}}) { |_i{{i}}|
      {% end %}
      {% for block_arg, i in block.args %}
        {{block_arg}} = _i{{i}}
      {% end %}
      {{block.body}}
      {% for arg in args %}
        }
      {% end %}
    {% end %}
  end
end

module Math
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "1.2.0") < 0 %}
    def isqrt(value : Int::Primitive)
      raise ArgumentError.new "Input must be non-negative integer" if value < 0
      return value if value < 2
      res = value.class.zero
      bit = res.succ << (res.leading_zeros_count - 2)
      bit >>= value.leading_zeros_count & ~0x3
      while (bit != 0)
        if value >= res + bit
          value -= res + bit
          res = (res >> 1) + bit
        else
          res >>= 1
        end
        bit >>= 2
      end
      res
    end
  {% end %}
end

macro min_u(a, b)
  {{a}} = Math.min({{a}}, {{b}})
end

macro max_u(a, b)
  {{a}} = Math.max({{a}}, {{b}})
end

macro zip(a, *b, &block)
  {{a}}.zip({{*b}}) {{block}}
end

class SegmentTree(T)
  def initialize(@n : Int32, @init : T = T.zero, &@compose : (T, T) -> T)
    @an = 1 << (@n - 1).bit_length
    @buf = Array.new(@an << 1, @init)
    propagate_all
  end

  def initialize(b : Array(T), @init : T = T.zero, &@compose : (T, T) -> T)
    @n = b.size
    @an = 1 << (@n - 1).bit_length
    @buf = Array.new(@an << 1, @init)
    @buf[@an, @n] = b
    propagate_all
  end

  def [](i : Int)
    @buf[i + @an]
  end

  def [](start : Int, count : Int)
    l, r = start + @an, start + count + @an
    r1 = r2 = @init
    while l != r
      if l.odd?
        r1 = @compose.call(r1, @buf[l])
        l += 1
      end
      if r.odd?
        r -= 1
        r2 = @compose.call(@buf[r], r2)
      end
      l >>= 1
      r >>= 1
    end
    @compose.call(r1, r2)
  end

  def [](r : Range)
    sc = Indexable.range_to_index_and_count(r, @n)
    raise ArgumentError.new("Invalid range") if sc.nil?
    self[*sc]
  end

  def []=(i : Int, v : T)
    @buf[i + @an] = v
    propagate(i + @an)
  end


  @an : Int32
  @buf : Array(T)

  private def propagate_all
    (1...@an).reverse_each do |i|
      @buf[i] = @compose.call(@buf[i << 1], @buf[i << 1 | 1])
    end
  end

  private def propagate(i : Int)
    while (i >>= 1) > 0
      @buf[i] = @compose.call(@buf[i << 1], @buf[i << 1 | 1])
    end
  end
end

solve(ProconIO.new)