/* -*- coding: utf-8 -*-
 *
 * 866.cc:  No.866 レベルKの正方形 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_H = 2000;
const int MAX_W = 2000;

/* typedef */

typedef long long ll;

/* global variables */

char s[MAX_W + 4];
int css[26][MAX_H + 1][MAX_W + 1];

/* subroutines */

inline int stlb(int i, int j, int maxl, int k) {
  int l0 = 0, l1 = maxl + 1;
  while (l0 + 1 < l1) {
    int l = (l0 + l1) / 2;

    int num = 0;
    for (int c = 0; c < 26; c++)
      if (css[c][i + l][j + l] - css[c][i + l][j] - css[c][i][j + l]
	  + css[c][i][j] > 0) num++;

    if (num >= k) l1 = l;
    else l0 = l;
  }
  return l1;
}

/* main */

int main() {
  int h, w, k;
  scanf("%d%d%d", &h, &w, &k);

  for (int i = 0; i < h; i++) {
    scanf("%s", s);
    for (int j = 0; j < w; j++) {
      int cij = s[j] - 'a';
      for (int c = 0; c < 26; c++)
	css[c][i + 1][j + 1] =
	  ((cij == c) ? 1 : 0)
	  + css[c][i + 1][j] + css[c][i][j + 1] - css[c][i][j];
    }
  }

  ll sum = 0;
  for (int i = 0; i < h; i++)
    for (int j = 0; j < w; j++) {
      int maxl = min(h - i, w - j);
      int l0 = stlb(i, j, maxl, k);
      if (l0 <= maxl) {
	int l1 = stlb(i, j, maxl, k + 1);
	sum += l1 - l0;
      }
    }

  printf("%lld\n", sum);
  return 0;
}