/* -*- 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 MAX_E2 = 1 << 12; // = 4096
const int BN = 26 / 2;
const int BBITS = 1 << BN;

/* typedef */

typedef long long ll;

template <typename T, const int MAX_E2>
struct SegTreeOr2D {
  int n, e2;
  T nodes[MAX_E2][MAX_E2];
  SegTreeOr2D() {}

  void init(int _n) {
    n = _n;
    for (e2 = 1; e2 < n; e2 <<= 1);
  }

  T &get(int i0, int i1) { return nodes[e2 - 1 + i0][e2 - 1 + i1]; }
  void set(int i0, int i1, T v) { get(i0, i1) = v; }

  void setall() {
    for (int j0 = e2 - 2; j0 >= 0; j0--) {
      int k00 = j0 * 2 + 1, k01 = k00 + 1;
      for (int j1 = e2 - 2; j1 >= 0; j1--) {
        int k10 = j1 * 2 + 1, k11 = k10 + 1;
        nodes[j0][j1] =
          nodes[k00][k10] | nodes[k00][k11] |
	  nodes[k01][k10] | nodes[k01][k11];
      }
    }
  }

  T or_range(int r00, int r01, int r10, int r11,
	     int k0, int k1, int i00, int i01, int i10, int i11) {
    if (r01 <= i00 || i01 <= r00 || r11 <= i10 || i11 <= r10)
      return 0;
    if (r00 <= i00 && i01 <= r01 && r10 <= i10 && i11 <= r11)
      return nodes[k0][k1];

    int im0 = (i00 + i01) / 2, im1 = (i10 + i11) / 2;
    int k00 = k0 * 2 + 1, k01 = k00 + 1;
    int k10 = k1 * 2 + 1, k11 = k10 + 1;

    T v00 = or_range(r00, r01, r10, r11, k00, k10, i00, im0, i10, im1);
    T v01 = or_range(r00, r01, r10, r11, k00, k11, i00, im0, im1, i11);
    T v10 = or_range(r00, r01, r10, r11, k01, k10, im0, i01, i10, im1);
    T v11 = or_range(r00, r01, r10, r11, k01, k11, im0, i01, im1, i11);
    return v00 | v01 | v10 | v11;
  }
  T or_range(int r00, int r01, int r10, int r11) {
    return or_range(r00, r01, r10, r11, 0, 0, 0, e2, 0, e2);
  }
};

/* global variables */

int bnums[BBITS];
SegTreeOr2D<int,MAX_E2> st;
char s[MAX_W + 4];

/* subroutines */

inline int bitnum(int bits) {
  return bnums[bits & (BBITS - 1)] + bnums[bits >> BN];
}

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;
    if (bitnum(st.or_range(i, i + l, j, j + l)) >= k) l1 = l;
    else l0 = l;
  }
  return l1;
}

/* main */

int main() {
  bnums[0] = 0;
  for (int bits = 1, msb = 1; bits < BBITS; bits++) {
    if ((msb << 1) <= bits) msb <<= 1;
    bnums[bits] = bnums[bits ^ msb] + 1;
  }

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

  st.init(max(h, w));

  for (int i = 0; i < h; i++) {
    scanf("%s", s);
    for (int j = 0; j < w; j++)
      st.set(i, j, 1 << (s[j] - 'a'));
  }
  st.setall();

  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;
}