#define rep(i, n) for(int i = 0; i < (int)(n); i++)
typedef long long ll;
#include <bits/stdc++.h>
using namespace std;
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll h,w,K;
    std::cin >> h>>w>>K;
    std::vector<string> s(h);
    // int sum[26][2001][2001];
    
    vector<vector<vector<int>>> sum(26,vector<vector<int>>(h+1,vector<int>(w+1)));
    for (int i = 0; i < h; i++) {
        std::cin >> s[i];
        for (int j = 0; j < w; j++) {
            int alpha = s[i][j]-'a';
            for (int k = 0; k < 26; k++) {
                sum[k][i+1][j+1] = sum[k][i][j+1] + sum[k][i+1][j] - sum[k][i][j];
            }
            sum[alpha][i+1][j+1]++;
        }
    }
    ll result = 0;
    // vector<vector<ll>> size_from(h,vector<ll>(w,1));
    
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            ll l,r;
            l = 0;
            r = min(h-i,w-j)+1;
            
            while(r-l>1){
                ll size = (r+l)/2;
                ll num = 0;
                for (int k = 0; k < 26; k++) {
                    ll cou = sum[k][i+size][j+size]-sum[k][i][j+size]-sum[k][i+size][j]+sum[k][i][j];
                    if(cou>0)num++;
                }
                if(num<K){
                    l = size;
                }else{
                    r = size;
                }
            }
            ll maxmin = l;
        
            r = min(h-i,w-j)+1;
            
            while(r-l>1){
                ll size = (r+l)/2;
                ll num = 0;
                for (int k = 0; k < 26; k++) {
                    ll cou = sum[k][i+size][j+size]-sum[k][i][j+size]-sum[k][i+size][j]+sum[k][i][j];
                    if(cou>0)num++;
                }
                if(num<=K){
                    l = size;
                }else{
                    r = size;
                }
            }
            
            result += l-maxmin;
        }
    }
    std::cout << result << std::endl;
}