#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(x) (x).begin(), (x).end()
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vector<int>>;
const int inf = 1e9;

int cs[26][2005][2005];
int ld[2005][2005];
int rd[2005][2005];

int cnt(int const i,int const j,int const d){
  int res=0;
  rep(a,26) if(cs[a][i][j]+cs[a][i+d][j+d]-cs[a][i+d][j]-cs[a][i][j+d]) res++;
  // cout<<i<<" "<<j<<" "<<d<<" "<<res<<" "<<endl;
  return res;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int h, w, k;
  cin >> h >> w >> k;
  rep(i, h){
    string s;
    cin >> s;
    rep(j,w) cs[s[j] - 'a'][i + 1][j + 1] = 1;
  }
  rep(a, 26) {
    rep(i, h) rep(j, w) cs[a][i + 1][j + 1] += cs[a][i + 1][j];
    rep(j, w) rep(i, h) cs[a][i + 1][j + 1] += cs[a][i][j + 1];
  }

  int ans = 0;
  rep(i, h) rep(j, w){
    int l,r;
    l=max(0,ld[i][j]-1);
    r=max(0,rd[i][j]-1);
    while(i+l<h && j+l<w && cnt(i,j,l+1)<k) l++;
    r=max(l,r);
    while(i+r<h && j+r<w && cnt(i,j,r+1)<=k) r++;
    ans+=r-l;
    // cout<<i<<" "<<j<<" "<<l<<" "<<r<<" "<<endl;
    ld[i+1][j+1]=l;
    rd[i+1][j+1]=r;
  }
  cout << ans << endl;
  return 0;
}