#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  int h, w, k; cin >> h >> w >> k;
  V<string> s(h); for (auto&& e : s) cin >> e;
  VV< array<int, 26> > c(h + 1, V< array<int, 26> >(w + 1));
  for (int i = h - 1; i >= 0; --i) for (int j = w - 1; j >= 0; --j) {
    for (int x = 0; x < 26; ++x) {
      c[i][j][x] = (s[i][j] - 'a' == x) + c[i + 1][j][x] + c[i][j + 1][x] - c[i + 1][j + 1][x];
    }
  }

  auto fn = [&](int i, int j, int x) -> int {
    auto chk = [&](int a) -> bool {
      int s = 0;
      for (int l = 0; l < 26; ++l) {
        s += !!(c[i][j][l] - c[i + a][j][l] - c[i][j + a][l] + c[i + a][j + a][l]);
      }
      return s <= x;
    };
    int ok = 0, ng = min(h - i, w - j) + 1;
    while (ng - ok > 1) {
      int mid = ok + ng >> 1;
      (chk(mid) ? ok : ng) = mid;
    }
    return ok;
  };
  
  lint res = 0;
  for (int i = 0; i < h; ++i) for (int j = 0; j < w; ++j) {
    res += fn(i, j, k) - fn(i, j, k - 1);
  }
  cout << res << '\n';
}