//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 100010001;
const ll LINF = (ll)INF*INF*10;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

const int N = 2010;

//head

int h, w, k;
string c[N];
int siz[N][N][27];
ll ans;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> h >> w >> k;
  rep(i, h) cin >> c[i];

  for(int i = h-1; i >= 0; i--) for(int j = w-1; j >= 0; j--) {
    rep(k, 26) {
      if(c[i][j] == 'a'+k) siz[i][j][k] = 1;
      else {
        if(i == h-1 or j == w-1) siz[i][j][k] = 2;
        else {
          siz[i][j][k] = min({h-i+1, w-j+1, siz[i+1][j][k], siz[i][j+1][k], siz[i+1][j+1][k]})+1;
        }
      }
    }
  }



  rep(i, h) rep(j, w) {
    sort(siz[i][j], siz[i][j]+26);
    ans += siz[i][j][k] - siz[i][j][k-1];
  }

  cout << ans << endl;
}