#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define each(x, y) for (auto &x : y)
#define all(a) (a).begin(), (a).end()
#define bit32(x) (1 << (x))
#define bit64(x) (1ll << (x))
#define sz(v) ((int) v.size())

using namespace std;

using i64 = long long;
using f80 = long double;
using vi32 = vector<int>;
using vi64 = vector<i64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

inline void yes() { cout << "Yes" << endl; exit(0); }
inline void no() { cout << "No" << endl; exit(0); }
inline i64 gcd(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); if (a % b == 0) return b; return gcd(b, a % b); }
inline i64 lcm(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); return a / gcd(a, b) * b; }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> inline void amax(T &x, T y) { x = max(x, y); }
template <typename T> inline void amin(T &x, T y) { x = min(x, y); }
template <typename T> inline T exp(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { each(x, v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }

int dp[26][2001][2001];

void solve() {
  int h, w, k;
  cin >> h >> w >> k;
  vstr c(h);
  cin >> c;
  rep(i, 26) repc(j, h) repc(k, w) {
    dp[i][j][k] = 1e9;
  }
  rep(x, 26) rrep(i, h) rrep(j, w) {
    char a = x + 'a';
    if (c[i][j] == a) {
      dp[x][i][j] = 1;
    } else {
      amin(dp[x][i][j], min({dp[x][i+1][j], dp[x][i][j+1], dp[x][i+1][j+1]}) + 1);
    }
  }
  i64 ans = 0;
  rep(i, h) rep(j, w) {
    vi32 v = {(int) 1e9};
    rep(x, 26) v.push_back(dp[x][i][j]);
    sort(all(v));
    if (v[k - 1] == 1e9) continue;
    if (v[k - 1] > h - i || v[k - 1] > w - j) continue;
    int min_size = v[k - 1];
    int max_size = min({ min(h - i, w - j) + 1, v[k] });
    ans += max(0, max_size - min_size);
  }
  cout << ans << endl;
}