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

using lint = long long;

constexpr int N = 2000;
int dss[N + 1][N + 1][27];

constexpr int INF = 1 << 30;

void solve() {
    int h, w, k;
    std::cin >> h >> w >> k;

    for (int i = 0; i <= h; ++i) {
        for (int j = 0; j <= w; ++j) {
            for (int d = 0; d < 27; ++d) {
                dss[i][j][d] = INF;
            }
        }
    }

    for (int i = 1; i <= h; ++i) {
        for (int j = 1; j <= w; ++j) {
            char c;
            std::cin >> c;
            dss[i][j][c - 'a'] = 0;

            for (int d = 0; d < 27; ++d) {
                dss[i][j][d] = std::min(dss[i][j][d],
                                        std::min({dss[i - 1][j][d],
                                                  dss[i][j - 1][d],
                                                  dss[i - 1][j - 1][d]}) +
                                            1);
            }
        }
    }

    lint ans = 0;
    for (int rx = 1; rx <= h; ++rx) {
        for (int ry = 1; ry <= w; ++ry) {
            auto ds = dss[rx][ry];
            std::sort(ds, ds + 27);

            int l = std::min(ds[k - 1], std::min(rx, ry));
            int r = std::min(ds[k], std::min(rx, ry));
            ans += r - l;
        }
    }

    std::cout << ans << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}