#include <iostream>
#include <algorithm>

using namespace std;

using ll = long long ;

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

int main() {
    cin >> h >> w >> k;

    char c;
    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'];
        }
    }

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

    cout << ans << endl;
}