#include <iostream>
#include <vector>
#include <algorithm>
#include <limits>

using namespace std;

int main() {
    int H, W, K;
    cin >> H >> W >> K;
    vector<vector<int>> A(H, vector<int>(W));
    for (int h = 0; h < H; ++h) {
        string row;
        cin >> row;
        for (int w = 0; w < W; ++w) {
            A[h][w] = row[w] - 'a';
        }
    }
    const int inf = 1<<30;
    vector<int> dp(26 * W, inf);
    long long ans = 0;
    for (int h = 0; h < H; ++h) {
        vector<int> prev = dp;
        fill(dp.begin(), dp.end(), inf);
        for (int w = 0; w < W; ++w) {
            for (int a = 0; a < 26; ++a) {
                if (A[h][w] == a) {
                    dp[a * W + w] = 0;
                }
                if (h > 0) {
                    dp[a * W + w] = min(dp[a * W + w], prev[a * W + w] + 1);
                }
                if (w > 0) {
                    dp[a * W + w] = min(dp[a * W + w], dp[a * W + w - 1] + 1);
                }
                if (h > 0 && w > 0) {
                    dp[a * W + w] = min(dp[a * W + w], prev[a * W + w - 1] + 1);
                }
            }
            vector<int> idx;
            for (int a = 0; a < 26; ++a) {
                if (dp[a * W + w] <= min(h, w)) {
                    idx.push_back(dp[a * W + w]);
                }
            }
            idx.push_back(min(h, w) + 1);
            if (idx.size() > K) {
                sort(idx.begin(), idx.end());
                ans += idx[K] - idx[K - 1];
            }
        }
    }
    cout << ans << endl;
    return 0;
}