#include <bits/stdc++.h>

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

int H, W, K;
vvi26 table;

int scaleCount(int, int, int);
int calcLevel(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 - 1}; h > 0; h--)
		ans += scaleCount(h, 0, K) - scaleCount(h, 0, K + 1);
	ans += scaleCount(0, 0, K) - scaleCount(0, 0, K + 1);
	for (int w{W}; w > 0; w--)
		ans += scaleCount(0, w, K) - scaleCount(0, w, K + 1);

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

	return 0;
}

int scaleCount(int begin_h, int begin_w, int K)
{
	int ans{};
	for (int right{1}, left{}; begin_h + right <= H && begin_w + right <= W; right++)
	{
		if (calcLevel(begin_h + left, begin_w + left, begin_h + right, begin_w + right) < K) continue;
		while (calcLevel(begin_h + left + 1, begin_w + left + 1, begin_h + right, begin_w + right) >= K)
			left++;
		ans += left + 1;
	}
	return ans;
}

int calcLevel(int top, int left, int bottom, int right)
{
	int level{};
	for (int c{}; c < 26; c++)
		if (table[bottom][right][c] + table[top][left][c] - table[top][right][c] - table[bottom][left][c] > 0)
			level++;
	return level;
}