#include <iostream>
#include <algorithm>
#include <vector>
#define inf 1000000000

using namespace std;

int h, w, k;
int sum[2005][2005];
char c[2005][2005];
int dp[2005][2005];
vector<int> vec[2005][2005];

int get(int x, int y, int k)
{
	return sum[x+k][y+k]-sum[x+k][y-1]-sum[x-1][y+k]+sum[x-1][y-1];
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> h >> w >> k;
	for(int y = 1; y <= h; y++){
		for(int x = 1; x <= w; x++){
			cin >> c[x][y];
		}
	}
	
	for(int x = 1; x <= w+1; x++) dp[x][h+1] = inf;
	for(int y = 1; y <= h+1; y++) dp[w+1][y] = inf;
	
	for(int i = 0; i < 26; i++){
		for(int x = 1; x <= w; x++){
			for(int y = 1; y <= h; y++){
				sum[x][y] = sum[x-1][y]+sum[x][y-1]-sum[x-1][y-1];
				if(c[x][y] == i+'a') sum[x][y]++;
			}
		}
		for(int y = h; y >= 1; y--){
			for(int x = w; x >= 1; x--){
				dp[x][y] = min(dp[x+1][y], dp[x][y+1]);
				if(c[x][y] == i+'a') dp[x][y] = 0;
				if(dp[x][y] >= inf) continue;
				if(get(x, y, dp[x][y]) == 0) dp[x][y]++;
				if(dp[x][y] > min(w-x, h-y)) continue;
				vec[x][y].push_back(dp[x][y]);
			}
		}
	}
	
	int ans = 0;
	for(int x = 1; x <= w; x++){
		for(int y = 1; y <= h; y++){
			if(vec[x][y].size() < k) continue;
			vec[x][y].push_back(min(w-x, h-y)+1);
			sort(vec[x][y].begin(), vec[x][y].end());
			ans += vec[x][y][k] - vec[x][y][k-1];
		}
	}
	cout << ans << endl;
	
	return 0;
}