#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); i++)
#define repr(i, n) for (int i = (n) - 1; i >= 0; i--)

using namespace std;
using ll = long long;

char S[2000][2001];
int sum[2001][2001][26];

int main() {
  int H, W, K;
  scanf("%d %d %d", &H, &W, &K);
  rep(i, H) scanf("%s", S[i]);
  rep(i, H) rep(j, W) rep(k, 26) {
    sum[i+1][j+1][k] = sum[i+1][j][k] + sum[i][j+1][k] - sum[i][j][k] + (S[i][j] - 'a' == k);
  }
  vector<int> from_y, from_x;
  from_y.push_back(0);
  from_x.push_back(0);
  rep(i, H-1) from_y.push_back(i+1), from_x.push_back(0);
  rep(i, W-1) from_y.push_back(0), from_x.push_back(i+1);
  auto calc = [&](int k, int y1, int y2, int x1, int x2) -> int {
    return sum[y2][x2][k] - sum[y2][x1][k] - sum[y1][x2][k] + sum[y1][x1][k];
  };
  ll ans = 0;
  rep(u, from_y.size()) {
    int y = from_y[u];
    int x = from_x[u];
    vector<int> ptr(26);
    int mn = 0;
    int mx = 0;
    int len = min(H - from_y[u], W - from_x[u]);
    for (; y < H && x < W; y++, x++) {
      rep(k, 26) {
        while (ptr[k] <= len && calc(k, y, ptr[k] + from_y[u], x, ptr[k] + from_x[u]) == 0) ptr[k]++;
      }
      auto count = [&](int x) {
        int ans = 0;
        rep(k, 26) ans += ptr[k] <= x;
        return ans;
      };
      while (mn <= len && count(mn) < K) mn++;
      while (mx <= len && count(mx) <= K) mx++;
      ans += mx - mn;
    }
  }
  cout << ans << endl;
}