//#define NDEBUG
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

constexpr usize operator"" _z(unsigned long long x) noexcept {
  return static_cast<usize>(x);
}

class rep {
  const usize f, l;

public:
  class itr {
    friend rep;
    usize i;
    constexpr itr(const usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr rep(const usize first, const usize last) noexcept
      : f(first), l(last) {}
  constexpr itr begin() const noexcept { return itr(f); }
  constexpr itr end() const noexcept { return itr(l); }
};
class revrep {
  const usize f, l;

public:
  class itr {
    friend revrep;
    usize i;
    constexpr itr(usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr revrep(usize first, usize last) noexcept : f(--first), l(--last) {}
  constexpr itr begin() const noexcept { return itr(l); }
  constexpr itr end() const noexcept { return itr(f); }
};
template <class T> using vec_alias = std::vector<T>;
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) {
  return a < b ? b - a : a - b;
}
template <class T> T scan() {
  T ret;
  std::cin >> ret;
  return ret;
}

} // namespace n91

#include <algorithm>
#include <array>
#include <iostream>
#include <utility>


namespace n91 {

void main_() {
  const usize h = scan<usize>();
  const usize w = scan<usize>();
  const usize k = scan<usize>();

  auto c = md_vec(h, w, usize());
  for (auto &v : c) {
    for (auto &e : v) {
      e = scan<char>() - 'a';
    }
  }

  auto sum = md_vec(h + 1, w + 1, std::array<u32, 26_z>({}));

  for (const auto i : rep(0_z, h)) {
    for (const auto j : rep(0_z, w)) {
      sum[i + 1_z][j + 1_z][c[i][j]] += 1_z;
    }
  }
  for (const auto i : rep(0_z, h)) {
    for (const auto j : rep(0_z, w)) {
      for (const auto k : rep(0_z, 26_z)) {
        sum[i + 1_z][j + 1_z][k] += sum[i + 1_z][j][k];
      }
    }
  }
  for (const auto i : rep(0_z, h)) {
    for (const auto j : rep(0_z, w)) {
      for (const auto k : rep(0_z, 26_z)) {
        sum[i + 1_z][j + 1_z][k] += sum[i][j + 1_z][k];
      }
    }
  }

  const auto count = [&](const usize xl, const usize xr, const usize yl,
                         const usize yr) {
    usize ret = 0_z;
    for (const auto i : rep(0_z, 26_z)) {
      if ((sum[xl][yl][i] + sum[xr][yr][i] - sum[xl][yr][i] - sum[xr][yl][i]) !=
          0_z) {
        ++ret;
      }
    }
    return ret;
  };

  u64 ans = static_cast<u64>(0);

  for (const auto xl : rep(0_z, h)) {
    for (const auto yl : rep(0_z, w)) {
      const usize maxd = std::min(h - xl, w - yl);
      usize low = 0_z, high = maxd + 1_z;
      while (high - low != 1_z) {
        const usize mid = (low + high) / 2_z;
        if (count(xl, xl + mid, yl, yl + mid) < k) {
          low = mid;
        } else {
          high = mid;
        }
      }
      const usize lb = high;
      low = 0_z, high = maxd + 1_z;
      while (high - low != 1_z) {
        const usize mid = (low + high) / 2_z;
        if (count(xl, xl + mid, yl, yl + mid) <= k) {
          low = mid;
        } else {
          high = mid;
        }
      }
      ans += high - lb;
    }
  }

  std::cout << ans << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}