#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <vector>
#define llint long long
#define inf 1000000000

using namespace std;

int h, w, k;
int sum[2005][2005];
char c[2005][2005];
int dp[2005][2005];
int vec[2005][2005][26];

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 y = h; y >= 1; y--){
			for(int x = w; x >= 1; x--){
				if(c[x][y] == i+'a') dp[x][y] = 0;
				else{
					dp[x][y] = dp[x+1][y];
					if(dp[x][y] > dp[x][y+1]) dp[x][y] = dp[x][y+1];
					if(dp[x][y] > dp[x+1][y+1]) dp[x][y] = dp[x+1][y+1];
					dp[x][y]++;
				}
				vec[x][y][i] = dp[x][y];
			}
		}
	}
	
	llint ans = 0;
	for(int x = 1; x <= w; x++){
		for(int y = 1; y <= h; y++){
			sort(vec[x][y], vec[x][y]+26);
			if(vec[x][y][k-1] > min(w-x, h-y)) continue;
			if(k >= 26 || vec[x][y][k] > min(w-x, h-y)) ans += min(w-x, h-y)+1;
			else ans += vec[x][y][k];
			ans -= vec[x][y][k-1];
		}
	}
	cout << ans << endl;
	
	return 0;
}