#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 counts[2001][2001][26];

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

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

    int kr = 1; // k を達成するのに最低限必要な半径
    while(kr*kr < k) ++kr;

    fill_n(counts[0][0], (h+1)*(w+1)*26, 0);
    for(int i=1;i<=h;++i) {
        for(int j=1;j<=w;++j) {
            for(int k=0;k<26;++k) {
                counts[i][j][k] = counts[i-1][j][k] + counts[i][j-1][k] - counts[i-1][j-1][k];
            }
            ++counts[i][j][a[i][j]];
        }
    }

    int ans = 0;
    for(int i=1;i<=h;++i) {
        for(int j=1;j<=w;++j) {
            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;
                }
            }
            int max_r_k = ok;

            ng = min(h, w);
            ok = 0;
            // ok: 半径 ok のとき level <= k-1;
            // ng: 半径 ng のとき　level > k-1;
            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-1) {
                    ng = r;
                } else {
                    ok = r;
                }
            }
            int max_r_k1 = ok;

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

    cout << ans << endl;
}