#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int cnt[26][2001][2001];

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int h, w, K;
    cin >> h >> w >> K;
    for (int i = 0; i < h; i++) {
        string s;
        cin >> s;
        for (int j = 0; j < w; j++) {
            for (int k = 0; k < 26; k++) {
                cnt[k][i + 1][j + 1] = cnt[k][i + 1][j];
            }
            cnt[s[j] - 'a'][i + 1][j + 1]++;
        }
    }

    for (int i = 1; i <= h; i++) {
        for (int j = 0; j <= w; j++) {
            for (int k = 0; k < 26; k++) {
                cnt[k][i][j] += cnt[k][i - 1][j];
            }
        }
    }

    ll ans = 0;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            int sz = min(i + 1, j + 1);

            int ng = 0, ok = sz + 1;
            while (ok - ng > 1) {
                int mid = (ok + ng) / 2;
                int val = 0;
                for (int k = 0; k < 26; k++) {
                    bool hoge = bool(cnt[k][i + 1][j + 1] + cnt[k][i + 1 - mid][j + 1 - mid] - cnt[k][i + 1][j + 1 - mid] - cnt[k][i + 1 - mid][j + 1]);
                    val += hoge;
                }
                if (val >= K) {
                    ok = mid;
                } else {
                    ng = mid;
                }
            }

            if (ok <= 0 || ok > sz) continue;

            int ng2 = sz + 1, ok2 = -1;
            while (ng2 - ok2 > 1) {
                int mid = (ok2 + ng2) / 2;
                int val = 0;
                for (int k = 0; k < 26; k++) {
                    bool hoge = bool(cnt[k][i + 1][j + 1] + cnt[k][i + 1 - mid][j + 1 - mid] - cnt[k][i + 1][j + 1 - mid] - cnt[k][i + 1 - mid][j + 1]);
                    val += hoge;
                }
                if (val <= K) {
                    ok2 = mid;
                } else {
                    ng2 = mid;
                }
            }

            if (ok2 <= 0 || ok2 > sz || ok > ok2) continue;
            ans += ok2 - ok + 1;
            //cerr << i << " " << j << ": " << ok << " " << ok2 << endl;
        }
    }
    cout << ans << "\n";
    return 0;
}