#include <iostream>
#include <algorithm>
using namespace std;
#pragma warning (disable: 4996)

long long H, W, K, ans; char c[2009][2009];
int p[2009][2009][27];

int main() {
	cin >> H >> W >> K;
	for (int i = 0; i < H; i++) scanf("%s", &c[i]);
	for (int t = 0; t < 27; t++) {
		for (int i = H; i >= 0; i--) {
			for (int j = W; j >= 0; j--) {
				if (c[i][j] == ('a' + t)) p[i][j][t] = 1;
				else p[i][j][t] = min({ (int)min(H - i, W - j) + 1, p[i][j + 1][t], p[i + 1][j][t], p[i + 1][j + 1][t] }) + 1;
			}
		}
	}
	for (int i = 0; i < H; i++) {
		for (int j = 0; j < W; j++) {
			sort(p[i][j], p[i][j] + 26);
			ans += 1LL * (p[i][j][K] - p[i][j][K - 1]);
		}
	}
	cout << ans << endl;
	return 0;
}