#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int wanted;
vector<vector<vector<int>>> c_sum;

int bin_lower(int x, int y, int max_size) {
    int ng = 0;
    int ok = max_size + 1;

    while (abs(ok - ng) > 1) {
        int mid = ng + (ok - ng) / 2;

        int count = 0;
        for (int i = 0; i < 26; i++) {
            if (c_sum[x + mid][y + mid][i] - c_sum[x + mid][y][i] - c_sum[x][y + mid][i] + c_sum[x][y][i] > 0)count++;
        }
        if (count >= wanted)ok = mid;
        else ng = mid;
    }

    return ok;
}

int bin_upper(int x, int y, int max_size) {
    int ng = 0;
    int ok = max_size + 1;

    while (abs(ok - ng) > 1) {
        int mid = ng + (ok - ng) / 2;

        int count = 0;
        for (int i = 0; i < 26; i++) {
            if (c_sum[x + mid][y + mid][i] - c_sum[x + mid][y][i] - c_sum[x][y + mid][i] + c_sum[x][y][i] > 0)count++;
        }
        if (count > wanted)ok = mid;
        else ng = mid;
    }

    return ok;
}

int main() {
    int h, w;
    cin >> h >> w >> wanted;
    vector<string> ban(h);
    for (int i = 0; i < h; i++)cin >> ban[i];

    c_sum = vector<vector<vector<int>>>(h + 1, vector<vector<int>>(w + 1, vector<int>(26)));
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            for (int c = 0; c < 26; c++) {
                c_sum[i + 1][j + 1][c] =
                        c_sum[i + 1][j][c] + c_sum[i][j + 1][c] - c_sum[i][j][c] + (ban[i][j] - 'a' == c ? 1 : 0);
            }
        }
    }

    int ret = 0;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            int max_size = min(h - i, w - j);
            int x = bin_upper(i, j, max_size);
            int y = bin_lower(i, j, max_size);
            ret += x - y;
        }
    }

    cout << ret << endl;
    return 0;
}