#include <bits/stdc++.h>
using namespace std;

int main() {
  ios::sync_with_stdio(false);

  int H, W, K;
  cin >> H >> W >> K;

  vector<string> G(H);
  for (int i = 0; i < H; ++i) {
    cin >> G[i];
  }

  vector<vector<vector<int>>> dp(26, vector<vector<int>>(H, vector<int>(W, 0x3f3f3f3f)));
  for (int i = H - 1; ~i; --i) {
    for (int j = W - 1; ~j; --j) {
      for (int c = 0; c < 26; ++c) {
        if (c == G[i][j] - 'a') {
          dp[c][i][j] = 1;
        } else {
          if (i + 1 < H && dp[c][i + 1][j] < 0x3f3f3f3f) {
            if (i + dp[c][i + 1][j] < H && j + dp[c][i + 1][j] < W) {
              dp[c][i][j] = min(dp[c][i][j], dp[c][i + 1][j] + 1);
            }
          }
          if (j + 1 < W && dp[c][i][j + 1] < 0x3f3f3f3f) {
            if (i + dp[c][i][j + 1] < H && j + dp[c][i][j + 1] < W) {
              dp[c][i][j] = min(dp[c][i][j], dp[c][i][j + 1] + 1);
            }
          }
          if (i + 1 < H && j + 1 < W) {
            dp[c][i][j] = min(dp[c][i][j], dp[c][i + 1][j + 1] + 1);
          }
        }
      }
    }
  }

  int64_t ans = 0;
  for (int i = 0; i < H; ++i) {
    for (int j = 0; j < W; ++j) {
      vector<int> ks;
      for (int c = 0; c < 26; ++c) {
        if (dp[c][i][j] < 0x3f3f3f3f) {
          ks.push_back(dp[c][i][j]);
        }
      }
      sort(ks.begin(), ks.end());
      if (K < ks.size()) {
        ans += ks[K] - ks[K - 1];
      } else if (K == ks.size()) {
        ans += 1 + min(H - i, W - j) - ks[K - 1];
      }

    }
  }

  cout << ans << endl;

  return 0;
}