#include <bits/stdc++.h>
#define all(vec) vec.begin(), vec.end()
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
constexpr ll INF = (1LL << 30) - 1LL;
constexpr ll LINF = (1LL << 60) - 1LL;
constexpr double eps = 1e-9;
constexpr ll MOD = 1000000007LL;
template <typename T> bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
};
template <typename T> bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
};
template <typename T> ostream &operator<<(ostream &os, vector<T> v) {
    for (int i = 0; i < v.size(); i++) {
        os << v[i] << (i + 1 == v.size() ? "\n" : " ");
    }
    return os;
}
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
    t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
    for (auto &e : t) {
        fill_v(e, v);
    }
};
int co[1 << 11][1 << 11][26];
int h, w, k;
int sum(int y1, int x1, int y2, int x2) {
    int res = 0;
    for (int k = 0; k < 26; k++) {
        if (co[y2][x2][k] - co[y1][x2][k] - co[y2][x1][k] + co[y1][x1][k] > 0) {
            res++;
        }
    }
    return res;
}
ll calc(int p) {
    ll res = 0;
    for (int i = 1; i < h; i++) {
        int y = i, x = 0;
        ll l = 0;
        while (x < w && y < h) {
            while (l <= x && sum(i + l, l, y + 1, x + 1) >= p) {
                l++;
            }
            res += l;
            x++;
            y++;
        }
    }
    for (int i = 0; i < w; i++) {
        int y = 0, x = i;
        int l = 0;
        while (x < w && y < h) {
            while (l <= y && sum(l, i + l, y + 1, x + 1) >= p) {
                l++;
            }
            res += l;
            x++;
            y++;
        }
    }
    return res;
}
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> h >> w >> k;
    string s;
    for (int i = 0; i < h; i++) {
        cin >> s;
        for (int j = 0; j < w; j++) {
            co[i + 1][j + 1][s[j] - 'a']++;
        }
    }
    for (int i = 0; i <= h; i++) {
        for (int j = 1; j <= w; j++) {
            for (int k = 0; k < 26; k++) {
                co[i][j][k] += co[i][j - 1][k];
            }
        }
    }
    for (int j = 0; j <= w; j++) {
        for (int i = 1; i <= h; i++) {
            for (int k = 0; k < 26; k++) {
                co[i][j][k] += co[i - 1][j][k];
            }
        }
    }
    cout << calc(k) - calc(k + 1) << '\n';
}