import java.io.IOException;
import java.io.InputStream;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.function.IntPredicate;


public class Main implements Runnable{

	public static void main(String[] args) {
		new Thread(null,new Main(), "" ,Runtime.getRuntime().maxMemory()).start();
	}
	
	public void run() {
		FastScanner sc=new FastScanner();
		int H=sc.nextInt();
		int W=sc.nextInt();
		int K=sc.nextInt();
		char[][] map=new char[H][W];
		int[][][] cnt=new int[26][H+2][W+2];
		int INF=Integer.MAX_VALUE/3;
		for(int i=0;i<H;++i) map[i]=sc.next().toCharArray();
		for(int v=0;v<26;++v) {
			for(int i=1;i<=H;++i) {
				for(int j=1;j<=W;++j) {
					cnt[v][i][j]=cnt[v][i][j-1];
					if((int)(map[i-1][j-1]-'a')==v) ++cnt[v][i][j];
				}
				for(int j=1;j<=W;++j) cnt[v][i][j]+=cnt[v][i-1][j];
			}
		}
		
		for(int v=0;v<26;++v) {
			for(int i=0;i<H+2;++i)
				cnt[v][i][W+1]=INF;
			for(int i=0;i<W+2;++i)
				cnt[v][H+1][i]=INF;
		}
		long ans=0;
		for(int i=1;i<=H;++i) {
			for(int j=1;j<=W;++j) {
				int lower=Math.min(H-(i-1),W-(j-1))+1;
				int upper=lower;//>=K
				int other=0;
				while(upper-other>1) {
					int middle=(other+upper)/2;
					int sum=0;
					for(int v=0;v<26;++v) {
						int p=i+middle-1;
						int q=j+middle-1;
						int r=i-1;
						int t=j-1;
						int a=(p>H||q>W)?INF:(cnt[v][p][q]-cnt[v][r][q]-cnt[v][p][t]+cnt[v][r][t]);
						assert(a>=0);
						if(a==INF) sum=INF;
						else if(a>0) ++sum;
					}
					if(sum>K) upper=middle;
					else other=middle;
				}
				other=0;
				while(lower-other>1) {
					int middle=(other+lower)/2;
					int sum=0;
					for(int v=0;v<26;++v) {
						int p=i+middle-1;
						int q=j+middle-1;
						int r=i-1;
						int t=j-1;
						int a=(p>H||q>W?INF:(cnt[v][p][q]-cnt[v][r][q]-cnt[v][p][t]+cnt[v][r][t]));
						assert(a>=0);
						if(a==INF) sum=INF;
						else if(a>0) ++sum;
					}
					if(sum>=K) lower=middle;
					else other=middle;
				}
				ans+=upper-lower;
			}
		}
		System.out.println(ans);
	}
	
	
	void tr(Object...objects) {System.out.println(Arrays.deepToString(objects));}
}

class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    
    public double nextDouble() { return Double.parseDouble(next());}
}