#include <bits/stdc++.h>

using namespace std;
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < (n); i++)
#define endl "\n"
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){os << "["; for (const auto &v : vec) {os << v << ","; } os << "]"; return os; }
template <typename T, typename U> ostream &operator<<(ostream &os, const pair<T, U> &p) {os << "(" << p.first << ", " << p.second << ")"; return os;}

void solve() {
    int H, W, K;
    cin >> H >> W >> K;
    vector<string> C(H);
    for (int i = 0; i < H; i++) {
        cin >> C[i];
    }
    vector<vector<vector<int>>> flag(26, vector<vector<int>>(H, vector<int>(W, 0)));
    vector<vector<vector<int>>> acc(26, vector<vector<int>>(H + 1, vector<int>(W + 1, 0)));

    for (int c = 0; c < 26; c++) {
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                if (C[i][j] == 'a' + c) {
                    flag[c][i][j] = 1;
                }
            }
        }
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                acc[c][i + 1][j + 1] = acc[c][i][j + 1] + acc[c][i + 1][j] - acc[c][i][j] + flag[c][i][j];
            }
        }
    }

    int ans = 0;
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            
            int tmax = min(H - i, W - j) + 1;
            int tmin = 0;
            while(tmax - tmin > 1) {
                int t = (tmax + tmin) / 2;
                int sum = 0;
                for(int c = 0; c < 26; c++) {
                    sum += (acc[c][i + t][j + t] - acc[c][i][j + t] - acc[c][i + t][j] + acc[c][i][j] > 0) ? 1 : 0;
                }
                if (sum <= K) {
                    tmin = t;
                } else {
                    tmax = t;
                }
            }
            int alpha = tmin;
            tmax = min(H - i, W - j) + 1;
            tmin = 0;
            while(tmax - tmin > 1) {
                int t = (tmax + tmin) / 2;
                int sum = 0;
                for(int c = 0; c < 26; c++) {
                    sum += (acc[c][i + t][j + t] - acc[c][i][j + t] - acc[c][i + t][j] + acc[c][i][j] > 0) ? 1 : 0;
                }
                if (sum < K) {
                    tmin = t;
                } else {
                    tmax = t;
                }
            }
            int beta = tmin;
            
            ans += alpha - beta;
        }
    }
    cout << ans << endl;

}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(16);
    solve();
    return 0;
}