#include <string>
#include <iostream>
#include <algorithm>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
const int inf = 1012345678;
int H, W, K, dp[2048][2048][32]; char S[2048][2048];
int main() {
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	cin >> H >> W >> K;
	for (int i = 0; i < H; ++i) {
		cin >> S[i];
	}
	for (int i = 0; i < H; ++i) {
		for (int j = 0; j < W; ++j) {
			int c = S[i][j] - 'a';
			for (int k = 0; k < c; ++k) {
				dp[i + 1][j + 1][k] = min({ dp[i + 1][j][k], dp[i][j + 1][k], dp[i][j][k] }) + 1;
			}
			for (int k = c + 1; k <= 26; ++k) {
				dp[i + 1][j + 1][k] = min({ dp[i + 1][j][k], dp[i][j + 1][k], dp[i][j][k] }) + 1;
			}
		}
	}
	long long ans = 0;
	for (int i = 1; i <= H; ++i) {
		for (int j = 1; j <= W; ++j) {
			sort(dp[i][j], dp[i][j] + 27);
			ans += dp[i][j][K] - dp[i][j][K - 1];
		}
	}
	cout << ans << endl;
	return 0;
}