#include <bits/stdc++.h>
#include <unistd.h>

using namespace std;

#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}


typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { -1,0, 1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
// #define mp make_pair
//#define endl '\n'

const int H = 200, W = 2000;
vector<vii> sum(H + 1, vii(W + 1, vi(26)));
int h, w, k;

bool check1(int i, int j, int ok) {
  //DEBUG(pii(i, j));
  //DEBUG(ok);
  int cnt = 0;
  vi num(26);
  rep (t, 26) {
    num[t] = sum[i + ok][j + ok][t] - sum[i][j + ok][t] - sum[i + ok][j][t] + sum[i][j][t];
    if (num[t] > 0) cnt++;
  }
  //DEBUG_VEC(num);
  return cnt >= k;
}

bool check2(int i, int j, int ok) {
  //DEBUG(pii(i, j));
  //DEBUG(ok);
  int cnt = 0;
  vi num(26);
  rep (t, 26) {
    num[t] = sum[i + ok][j + ok][t] - sum[i][j + ok][t] - sum[i + ok][j][t] + sum[i][j][t];
    if (num[t] > 0) cnt++;
  }
  //DEBUG_VEC(num);
  return cnt > k;
}


int nibutan(int i, int j) {
  int ok = min(w - j, h - i);
  if (not check1(i, j, ok)) return 0;

  int ng = 0;
  while (ng + 1 < ok) {
    int mid = (ng + ok) / 2;
    if (check1(i, j, mid)) {
      ok = mid;
    }
    else {
      ng = mid;
    }
  }

  int low_ok = ok;
  ok = min(w - j, h - i);
  if (not check2(i, j, ok)) {
    return ok - low_ok + 1;
  }

  ng = low_ok - 1;
  while (ng + 1 < ok) {
    int mid = (ng + ok) / 2;
    if (check2(i, j, mid)) {
      ok = mid;
    }
    else {
      ng = mid;
    }
  }
  //DEBUG(low_ok);
  //DEBUG(ok);
  return ok - low_ok;
}

int main() {
  cin >> h >> w >> k;
  vector<vii> num(h, vii(w, vi(26)));
  rep (i, h) {
    rep (j, w) {
      char c;
      cin >> c;
      num[i][j][c - 'a']++;
    }
  }

  rep (i, h) {
    rep (j, w) {
      rep (k, 26) {
        sum[i + 1][j + 1][k] = sum[i + 1][j][k] + num[i][j][k];
      }
    }
    rep (j, w + 1) {
      rep (k, 26) {
        sum[i + 1][j][k] += sum[i][j][k];
      }
    }
  }

  ll ans = 0;
  rep (i, h) {
    rep (j, w) {
      //DEBUG(pii(i, j));
      ans += nibutan(i, j);
      //DEBUG(ans);
    }
  }
  cout << ans << endl;
}