#define rep(i, n) for(int i = 0; i < (int)(n); i++)
typedef int 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);
    // ll sum[26][2001][2001];
    vector<vector<vector<ll>>> sum(26,vector<vector<ll>>(h+1,vector<ll>(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 = 1;
            ll r = min(h-i,w-j);
            
            // numより大きいなら小さく
            // numより小さいなら大きく
            
            // K未満の最大値
            if(K==1){
                l = 0;
            }else{
                while(r-l>1){
                    ll m = (r+l)/2;
                    ll num = 0;
                    for (int k = 0; k < 26; k++) {
                        ll cou = sum[k][i+m+1][j+m+1]-sum[k][i][j+m+1]-sum[k][i+m+1][j]+sum[k][i][j];
                        if(cou>0)num++;
                    }
                    if(num<K){
                        l = m;
                    }else{
                        r = m;
                    }
                }
            }
            
            ll maxmiman = l;
            r = min(h-i,w-j);
            
            while(r-l>1){
                ll m = (r+l)/2;
                ll num = 0;
                for (int k = 0; k < 26; k++) {
                    ll cou = sum[k][i+m+1][j+m+1]-sum[k][i][j+m+1]-sum[k][i+m+1][j]+sum[k][i][j];
                    if(cou>0)num++;
                }
                if(num <= K){
                    l = m;
                }else{
                    r = m;
                }
            }
            result += l-maxmiman;
            // Kである最大値。
            
        }
    }
    std::cout << result << std::endl;
}