package contest190816;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.Random;

public class D2 {
	InputStream is;
	PrintWriter out;
	String INPUT = "";
	
	void solve()
	{
		int n = ni(), m = ni(), K = ni();
		char[][] map = nm(n, m);
		
		int I = 999999999;
		int[][][] all = new int[26][][];
		for(int u = 0;u < 26;u++){
			int[][] dp = new int[n][m];
			for(int i = 0;i < n;i++){
				for(int j = 0;j < m;j++){
					if(map[i][j]-'a' == u){
						dp[i][j] = 0;
					}else{
						dp[i][j] = I;
					}
				}
			}
			for(int i = n-1;i >= 0;i--){
				for(int j = m-1;j >= 0;j--){
					if(i+1 < n){
						dp[i][j] = Math.min(dp[i][j], dp[i+1][j] + 1);
					}
					if(j+1 < m){
						dp[i][j] = Math.min(dp[i][j], dp[i][j+1] + 1);
					}
					if(i+1 < n && j+1 < m){
						dp[i][j] = Math.min(dp[i][j], dp[i+1][j+1] + 1);
					}
				}
			}
			
			all[u] = dp;
		}
		
		long ret = 0;
		int[] temp = new int[26];
		Random gen = new Random();
		for(int i = 0;i < n;i++){
			for(int j = 0;j < m;j++){
				for(int k = 0;k < 26;k++){
					temp[k] = all[k][i][j];
				}
				int upper = select(temp, K, 0, 26, gen);
				int lower = select(temp, K-1, 0, 26, gen);
//				Arrays.sort(temp);
//				int upper = temp[K];
//				int lower = temp[K-1];
				upper = Math.min(upper, n-i);
				upper = Math.min(upper, m-j);
				ret += Math.max(0, upper-lower);
			}
		}
		out.println(ret);
	}
	
	public static int select(int[] a, int K, int f, int t, Random gen)
	{
		if(f >= t)throw new IllegalArgumentException();
		if(K < 0 || K >= t-f)throw new IllegalArgumentException();
//		U.tr(a);
		while(t - f > 1){
			int pivot = a[gen.nextInt(t-f)+f];
//			U.tr(f, t, K, pivot);
			int i = f, j = t-1;
			while(true){
				while(i < j && a[i] < pivot)i++;
				while(i < j && a[j] >= pivot)j--;
				if(i < j){
					int d = a[i]; a[i] = a[j]; a[j] = d;
				}else{
					break;
				}
			}
			
			if(f+K < i){
				t = i;
			}else{
				K -= i-f;
				f = i;
				int k = i, l = t-1;
				while(true){
					while(k < l && a[k] == pivot)k++;
					while(k < l && a[l] > pivot)l--;
					if(k < l){
						int d = a[k]; a[k] = a[l]; a[l] = d;
					}else{
						break;
					}
				}
				if(f+K < k){
					return pivot;
				}else{
					K -= k-f;
					f = k;
				}
			}
//			U.tr(f, t, K, a, i);
		}
		return a[f];
	}
	
	void run() throws Exception
	{
//		int n = 2000, m = 99999;
//		Random gen = new Random();
//		StringBuilder sb = new StringBuilder();
//		sb.append(n + " ");
//		sb.append(n + " ");
//		sb.append(1 + " ");
//		for (int i = 0; i < n; i++) {
//			for(int j = 0;j < n;j++){
//				sb.append("a");
//			}
//			sb.append("\n");
//		}
//		INPUT = sb.toString();

		
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//		Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){
//			@Override
//			public void run() {
//				long s = System.currentTimeMillis();
//				solve();
//				out.flush();
//				if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//			}
//		};
//		t.start();
//		t.join();
	}
	
	public static void main(String[] args) throws Exception { new D2().run(); }
	
	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	
	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }
	
	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private long[] nal(int n)
	{
		long[] a = new long[n];
		for(int i = 0;i < n;i++)a[i] = nl();
		return a;
	}
	
	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private int[][] nmi(int n, int m) {
		int[][] map = new int[n][];
		for(int i = 0;i < n;i++)map[i] = na(m);
		return map;
	}
	
	private int ni() { return (int)nl(); }
	
	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); }
}