#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

const long long MAX = 5100000;
const long long INF = 1LL << 60;
//const long long mod = 1000000007LL;
//const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

int sum[26][2001][2001];
int main()
{
	
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	
	ll H, W, K; cin >> H >> W >> K;
	for (ll i = 0; i < H; i++) {
		string s; cin >> s;
		for (ll j = 0; j < W; j++) sum[s[j] - 'a'][i + 1][j + 1]++;
	}
	for (ll i = 0; i < 26; i++)for (ll j = 1; j <= H; j++)for (ll k = 1; k <= W; k++) sum[i][j][k] += sum[i][j][k - 1] + sum[i][j - 1][k] - sum[i][j - 1][k - 1];
	ll res = 0;
	for (ll i = 0; i < H; i++) {
		for (ll j = 0; j < W; j++) {
			ll low = 0;
			ll upp = 0;
			{
				ll left = 0;
				ll right = min(H, W);
				while (right - left > 1) {
					ll mid = (left + right) >> 1;
					if (i + mid - 1 >= H || j + mid - 1 >= W) {
						right = mid - 1;
						continue;
					}
					ll cnt = 0;
					for (ll k = 0; k < 26; k++) {
						cnt += ((sum[k][i + mid][j + mid] - sum[k][i][j + mid] - sum[k][i + mid][j] + sum[k][i][j]) > 0);
					}
					if (cnt < K) {
						left = mid;
					}
					else {
						right = mid;
					}
				}
				ll cnt = 0;
				for (ll k = 0; k < 26; k++) {
					cnt += (sum[k][i + right][j + right] - sum[k][i][j + right] - sum[k][i + right][j] + sum[k][i][j]) > 0;
				}
				if (cnt >= K) low = right;
				else low = right + 1;
			}
			{
				ll left = 0;
				ll right = min(H, W);
				while (right - left > 1) {
					ll mid = (left + right) >> 1;
					if (i + mid - 1 >= H || j + mid - 1 >= W) {
						right = mid - 1;
						continue;
					}
					ll cnt = 0;
					for (ll k = 0; k < 26; k++) {
						cnt += (sum[k][i + mid][j + mid] - sum[k][i][j + mid] - sum[k][i + mid][j] + sum[k][i][j]) > 0;
					}
					if (cnt <= K) {
						left = mid;
					}
					else {
						right = mid;
					}
				}
				ll cnt = 0;
				for (ll k = 0; k < 26; k++) {
					cnt += (sum[k][i + right][j + right] - sum[k][i][j + right] - sum[k][i + right][j] + sum[k][i][j]) > 0;
					//cout << i << " " << j << " " << k << " " << cnt << endl;
				}
				//cout << i << " " << j << " " << right << " " << cnt << endl;
				if (cnt > K) upp = right;
				else upp = right + 1;
			}
			//cout << i<<" "<<j<<" "<<low << " " << upp << endl;
			res += upp - low;
		}
	}
	cout << res << endl;
	return 0;
}