/* -*- 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;
const int INF = 1 << 29;

/* typedef */

typedef long long ll;

/* global variables */

char flds[MAX_H][MAX_W + 4];
int dp[MAX_H + 1][MAX_W + 1][26];

/* subroutines */

inline int min3(int a, int b, int c) {
  if (a < b) return (a < c) ? a : c;
  return (b < c) ? b : c;
}

/* main */

int main() {
  int h, w, k;
  scanf("%d%d%d", &h, &w, &k);
  for (int i = 0; i < h; i++) scanf("%s", flds[i]);

  for (int i = h; i >= 0; i--)
    for (int j = w; j >= 0; j--) {
      if (i == h || j == w)
	fill(dp[i][j], dp[i][j] + 26, INF);
      else {
	int cij = flds[i][j] - 'a';
	for (int c = 0; c < 26; c++) {
	  if (c == cij)
	    dp[i][j][c] = 1;
	  else
	    dp[i][j][c] =
	      min3(dp[i][j + 1][c], dp[i + 1][j][c], dp[i + 1][j + 1][c])
	      + 1;
	}
      }
    }

  ll sum = 0;
  for (int i = 0; i < h; i++)
    for (int j = 0; j < w; j++) {
      int *as = dp[i][j];
      sort(as, as + 26);

      int maxl = min(h - i, w - j);
      int a0 = as[k - 1];
      if (a0 <= maxl) {
	int a1 = min(maxl + 1, (k >= 26) ? INF : as[k]);
	sum += a1 - a0;
      }
    }

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