#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

int cnt[26][2001][2001];
int H, W, K;

bool exist(int h1, int w1, int h2, int w2, char c){
    return cnt[c-'a'][h2+1][w2+1]-cnt[c-'a'][h1][w2+1]-cnt[c-'a'][h2+1][w1]+cnt[c-'a'][h1][w1] > 0;
}

int sum(int h1, int w1, int h2, int w2){
    int ans = 0;
    for(int i = 0; i < 26; i++){
        if(exist(h1, w1, h2, w2, 'a'+i)) ans++;
    }
    return ans;
}

int search_upper(int h1, int w1, int m){
    if(sum(h1, w1, h1+1, w1+1) > m) return 0;
    int l = 1, r = min(H-h1-1, W-w1-1);
    if(sum(h1, w1, h1+r, w1+r) == m) return r;
    while(r-l > 1){
        int c = (l+r)/2;
        if(sum(h1, w1, h1+c, w1+c) > m) r = c;
        else l = c;
    }
    return l;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    cin >> H >> W >> K;
    for(int i = 0; i < H; i++){
        for(int j = 0; j < W; j++){
            char c;
            cin >> c;
            for(int k = 0; k < 26; k++){
                cnt[k][i+1][j+1] = cnt[k][i+1][j]+cnt[k][i][j+1]-cnt[k][i][j];
                if(c == 'a'+k) cnt[k][i+1][j+1]++;
            }
        }
    }
    ll ans = 0;
    for(int i = 0; i < H; i++){
        for(int j = 0; j < W; j++){
            ans += max(0, search_upper(i, j, K)-search_upper(i, j, K-1));
        }
    }
    cout << ans << endl;
}