#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cerr << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cerr<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 500010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 30;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
mt19937 rng; //use it by rng() % mod, shuffle(all(vec), rng)
///////////////////////////////////////////////////////////////////////////////////////////////////

struct Ruiseki2D {
	int N, M;
	vector<vi> B;
	void init(int n, int m) {
		N = n; M = m;
		B = vector<vi>(N + 1, vi(M + 1, 0));
	}
	Ruiseki2D() {}
	Ruiseki2D(int n, int m) { init(n, m); }
	void add(int a, int b, int v) {
		if(a < 0 || N <= a || b < 0 || M <= b) return;
		B[a + 1][b + 1] += v;
	}
	void sweep() {
		rep(i, 1, N + 1)
			rep(j, 1, M + 1)
				B[i][j] += B[i][j - 1] + B[i - 1][j] - B[i - 1][j - 1];
	}
	ll get(int a, int b, int c, int d) { //[(a, b), (c, d))
		if(a >= c || b >= d) return 0;
		a = max(a, 0); b = max(b, 0); c = min(c, N); d = min(d, M);
		return B[c][d] - B[c][b] - B[a][d] + B[a][b];
	}
};

int N, M, K;
int R[26][2010][2010];
char A[2010][2010];

int dx[3] = {-1, 0, -1};
int dy[3] = {0, -1, -1};


void solve() {
	cin >> N >> M >> K;
	rep(i, 0, N) {
		rep(j, 0, M) {
			cin >> A[i][j];
		}
	}
	rep(k, 0, 26) {
		queue<pi> que;
		rep(i, 0, N + 1) {
			rep(j, 0, M + 1) {
				if(i < N && j < M) {
					if(A[i][j] == 'a' + k) {
						R[k][i][j] = 1;
						que.push(pi(i, j));
					}
					else R[k][i][j] = inf;
				}
				else {
					R[k][i][j] = 1;
					que.push(pi(i, j));
				}
			}
		}
		while(!que.empty()) {
			pi p = que.front(); que.pop();
			int x = p.fst, y = p.sec;
			int c = R[k][x][y];
			rep(i, 0, 3) {
				int nx = dx[i] + x, ny = dy[i] + y;
				if(0 <= nx && nx <= N && 0 <= ny && ny <= M) {
					if(R[k][nx][ny] == inf) {
						R[k][nx][ny] = c + 1;
						que.push(pi(nx, ny));
					}
				}
			}
		}
	}
	// rep(k, 0, 8) {
	// 	rep(i, 0, N) {
	// 		debug(vi(R[k][i], R[k][i] + M));
	// 	}
	// 	debug("---------");
	// }
	auto get = [&](int i, int j) {
		int rv = min(N - i, M - j) + 1;
		vi vec(26);
		rep(k, 0, 26) vec[k] = R[k][i][j];
		vec.pb(rv);
		sort(all(vec));
		// debug(vec);
		return vec[K] - vec[K - 1];
	};
	ll res = 0;
	rep(i, 0, N) {
		rep(j, 0, M) {
			res += get(i, j);
		}
	}
	cout << res << "\n";
}

uint32_t rd() {
	uint32_t res;
#ifdef __MINGW32__
	asm volatile("rdrand %0" :"=a"(res) ::"cc");
#else
	res = std::random_device()();
#endif
	return res;
}

int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
    cerr << fixed;
	cerr.precision(6);
	rng.seed(rd());
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
	if(!freopen("in.txt", "rt", stdin)) return 1;
#endif	
	solve();
    cerr << "Time: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
	return 0;
}