#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target ("avx")

#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> void chmin(T &t, const T &f) { if (t > f) t = f; }
template <class T> void chmax(T &t, const T &f) { if (t < f) t = f; }

int H, W, K;
char C[2010][2010];

int sum[2010][2010][26];

inline int calc(int x0, int x1, int y0, int y1, int e) {
  return sum[x0][y0][e] - sum[x0][y1][e] - sum[x1][y0][e] + sum[x1][y1][e];
}
inline int get(int x, int y, int l) {
  int ret = 0;
  for (int e = 0; e < 26; ++e) {
    if (calc(x, x + l, y, y + l, e) > 0) {
      ++ret;
    }
  }
  return ret;
}

int main() {
  for (; ~scanf("%d%d%d", &H, &W, &K); ) {
    for (int x = 0; x < H; ++x) {
      scanf("%s", C[x]);
    }
    
    for (int x = 0; x < H; ++x) for (int y = 0; y < W; ++y) {
      for (int e = 0; e < 26; ++e) {
        sum[x + 1][y + 1][e] = sum[x + 1][y][e] + sum[x][y + 1][e] - sum[x][y][e] + ((C[x][y] - 'a' == e) ? 1 : 0);
      }
    }
    
    Int ans = 0;
    for (int s = 0; s < 2; ++s) {
      for (int x = 0; x < H; ++x) {
        int l = 1;
        for (int y = 0; y < W; ++y) {
          --l;
          for (; x + l <= H && y + l <= W && get(x, y, l) < K + s; ++l) {}
          ans += (s == 0) ? -l : +l;
        }
      }
    }
    printf("%lld\n", ans);
  }
  return 0;
}