#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <stdio.h>
#include <vector>
#include <algorithm>
using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
#define FOR(i,a,n) for(int i=(a);i<(n);++i)
#define each(i, a) for(auto &i : a)
template<class T>inline vector<T> vec(size_t a) { return vector<T>(a); }
template<class T, class... Ts>inline auto vec(size_t a, Ts... ts) { return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...)); }

template<class T> class acc2d {
    int h, w;
    vector<vector<T>>& dat;
public:
    acc2d(vector<vector<T>>& _dat) : dat(_dat), h(_dat.size()), w(_dat[0].size()) {
        FOR(i, 0, h)FOR(j, 1, w)dat[i][j] += dat[i][j - 1];
        FOR(i, 1, h)FOR(j, 0, w)dat[i][j] += dat[i - 1][j];
    }
    T sum(int y1, int x1, int y2, int x2) {
        T ret = dat[y2][x2];
        if (0 < y1)ret -= dat[y1 - 1][x2];
        if (0 < x1)ret -= dat[y2][x1 - 1];
        if (0 < y1 && 0 < x1)ret += dat[y1 - 1][x1 - 1];
        return ret;
    }
    T sum(int y2, int x2) { return sum(0, 0, y2, x2); }
};

int main() {
    int h, w, k; scanf("%d %d %d", &h, &w, &k);
    auto dat = vec<int>(26, h, w);
    FOR(y, 0, h) {
        char s[20]; scanf("%s", s);
        FOR(x, 0, w)++dat[s[x] - 'a'][y][x];
    }
    vector<acc2d<int>> acc;
    each(i, dat)acc.emplace_back(i);

    ll ans = 0;
    FOR(y, 0, h)FOR(x, 0, w) {
        int l, r;
        {
            int ok = 0, ng = min(h - y, w - x) + 1;
            while (ng - ok > 1) {
                int mid = ok + ng >> 1;
                int kind = 0;
                FOR(i, 0, 26) {
                    if (acc[i].sum(y, x, y + mid - 1, x + mid - 1))++kind;
                }
                if (kind <= k)ok = mid;
                else ng = mid;
            }
            r = ok;
        }
        {
            int ok = 0, ng = min(h - y, w - x) + 1;
            while (ng - ok > 1) {
                int mid = ok + ng >> 1;
                int kind = 0;
                FOR(i, 0, 26) {
                    if (acc[i].sum(y, x, y + mid - 1, x + mid - 1))++kind;
                }
                if (kind < k)ok = mid;
                else ng = mid;
            }
            l = ok;
        }
        ans += r - l;
    }
    printf("%lld\n", ans);

    return 0;
}