#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (ll i=(ll)from; i<(ll)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }


int main() {
  ll H, W, K; cin >> H >> W >> K;
  string c[H]; rep(i,H) cin >> c[i];
  int s[H][W][26] = {}; rep(i,H) rep(j,W) fill_n(s[i][j],26,INF);

  queue<p_ll> q[26];
  rep(i,H) rep(j,W) {
    ll num = c[i][j]-'a';
    s[i][j][num] = 1;
    q[num].push(make_pair(i,j));
  }
  rep(i,26) {
    // cout << (char)('a'+i) << endl;
    while (q[i].size()) {
      p_ll now = q[i].front(); q[i].pop();
      ll nx = now.first, ny = now.second;
      // cout << now.first << "," << now.second << endl;
      if (nx!=0&&s[nx-1][ny][i]==INF) {
        s[nx-1][ny][i] = s[nx][ny][i] + 1;
        q[i].push(make_pair(nx-1,ny));
      }
      if (ny!=0&&s[nx][ny-1][i]==INF) {
        s[nx][ny-1][i] = s[nx][ny][i] + 1;
        q[i].push(make_pair(nx,ny-1));
      }
      if (nx!=0&&ny!=0&&s[nx-1][ny-1][i]==INF) {
        s[nx-1][ny-1][i] = s[nx][ny][i] + 1;
        q[i].push(make_pair(nx-1,ny-1));
      }
    }
  }
  // rep(k,3) { rep(i,H) { rep(j,W) cout << s[i][j][k] << " "; cout << endl; } cout << endl; }

  ll result = 0;
  rep(i,H) rep(j,W) {
    sort(s[i][j],s[i][j]+26);
    ll plus = s[i][j][K-1]<=min(H-i,W-j) ? min(min(H-i,W-j),(K==26?LLINF:s[i][j][K]-1))-s[i][j][K-1]+1 : 0;
    result += plus;
    // cout << i << "," << j << "->" << plus << endl;
    // debug(s[i][j],s[i][j]+26);
  }
  cout << result << endl;
  return 0;
}