#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(x) (x).begin(), (x).end()
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vector<int>>;
const int inf = 1e9;
const ll lim = 1e18;

int t[12][2005][2005];
int h, w, k;

int bitsize(int a) {
  int res = 0;
  while (a) {
    res++;
    a >>= 1;
  }
  return res;
}

int cnt(int i, int l, int r) {
  int d = r - l + 1;
  if (i + d > h) return inf;
  if (l == r) return 1;
  int e = bitsize(d - 1) - 1;
  int m = d - (1 << e);
  int s = t[e][i][l] | t[e][i + m][l] | t[e][i][l + m] | t[e][i + m][l + m];
  return __builtin_popcount(s);
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> h >> w >> k;

  int mxe = bitsize(min(h, w)) - 1;

  rep(i, h) {
    string s;
    cin >> s;
    rep(j, w) t[0][i][j] = 1 << (s[j] - 'a');
  }

  rep(e, mxe) {
    int d = 1 << e;
    rep(i, h) {
      if (i + d * 2 > h) break;
      rep(j, w) {
        if (j + d * 2 > w) break;
        t[e + 1][i][j] =
            t[e][i][j] | t[e][i + d][j] | t[e][i][j + d] | t[e][i + d][j + d];
      }
    }
  }

  ll ans = 0;
  rep(i, h) {
    int r0 = 0, r1 = 0;
    rep(l, w) {
      if (r0 < l) r0 = l;
      while (r0 < w && cnt(i, l, r0) < k) r0++;
      if (r0 == w) break;
      if (r1 < r0) r1 = r0;
      while (r1 < w && cnt(i, l, r1) <= k) r1++;
      ans += r1 - r0;
    }
  }

  cout << ans << endl;
  return 0;
}