#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cassert>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
#include <stack>
#include <queue>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <bitset>

using namespace std;

using ll =  long long;
using Pll = pair<ll, ll>;
using Pii = pair<int, int>;

constexpr ll MOD = 1000000007;
constexpr long double EPS = 1e-10;
constexpr int dyx[4][2] = {
    { 0, 1}, {-1, 0}, {0,-1}, {1, 0}
};

int h,w,k;
int counts[26][2001][2001];

int count_level(int i, int j, int r) {
    int level = 0;
    for(int k=0;k<26;++k) {
        if(counts[k][i+r-1][j+r-1] - counts[k][i+r-1][j-1] - counts[k][i-1][j+r-1] + counts[k][i-1][j-1] > 0) {
            ++level;
            if(level > k) break;
        }
    }
    return level;
}

int binary_search(int i, int j, int k) {
    int ok = 0, ng = min(h, w);
    // ok: 半径 ok のとき level <= k;
    // ng: 半径 ng のとき　level > k;
    while(ng-ok > 1) {
        int r = (ok+ng) / 2;
        if(i+r-1 > h || j+r-1 > w) {
            ng = r;
            continue;
        }
        int level = count_level(i, j, r);
        if(level > k) {
            ng = r;
        } else {
            ok = r;
        }
    }
    return ok;
}

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

    int ans = 0;
    for(int i=1;i<=h;++i) {
        for(int j=1;j<=w;++j) {
            int max_r_k = binary_search(i, j, k);
            int max_r_k1 = binary_search(i, j, k-1);

            ans += max(0, max_r_k-max_r_k1);
            // cerr << i << "," << j << ": " << max_r_k << " " << max_r_k1 << endl;
        }
    }

    cout << ans << endl;
}