#include <iostream>
#include <algorithm>

using namespace std;

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

inline 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;
}

inline 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;

    char c;
    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];
            }
            cin >> c;
            ++counts[i][j][c-'a'];
        }
    }

    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;
}