#include <bits/stdc++.h>

using vi26 = std::vector<std::array<int, 26>>;
using vvi26 = std::vector<vi26>;

int H, W, K;
vvi26 table;

int calcLevelK(int, int);
bool lowerBound(int, int, int, int);

int main()
{
	scanf("%d%d%d", &H, &W, &K);
	table.resize(H + 1, vi26(W + 1));
	for (int h{1}; h <= H; h++)
		for (int w{1}; w <= W; w++)
		{
			char c;
			scanf(" %c", &c);
			table[h][w][c - 'a']++;
		}
	for (int h{1}; h <= H; h++)
		for (int w{1}; w <= W; w++)
			for (int c{}; c < 26; c++)
				table[h][w][c] += table[h][w - 1][c];
	for (int h{1}; h <= H; h++)
		for (int w{}; w <= W; w++)
			for (int c{}; c < 26; c++)
				table[h][w][c] += table[h - 1][w][c];
	int64_t ans{};
	for (int h{}; h < H; h++)
		for (int w{}; w < W; w++)
			ans += calcLevelK(h, w);

	printf("%lld\n", ans);

	return 0;
}

int calcLevelK(int h, int w)
{
	int k_left{}, k_right{std::min(H + 1 - h, W + 1 - w)};
	while (k_right - k_left > 1)
	{
		int mid{(k_right + k_left) / 2};
		if (lowerBound(h, w, mid, K)) k_right = mid;
		else k_left = mid;
	}
	if (k_right == std::min(H + 1 - h, W + 1 - w)) return 0;
	int k1_left{}, k1_right{std::min(H + 1 - h, W + 1 - w)};
	while (k1_right - k1_left > 1)
	{
		int mid{(k1_right + k1_left) / 2};
		if (lowerBound(h, w, mid, K + 1)) k1_right = mid;
		else k1_left = mid;
	}
	return k1_right - k_right;
}

bool lowerBound(int h, int w, int size, int k)
{
	int level{};
	for (int c{}; c < 26; c++)
		if (table[h + size][w + size][c] + table[h][w][c] - table[h][w + size][c] - table[h + size][w][c] > 0)
			level++;
	return level >= k;
}