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


public class Main{

	public static void main(String[] args) {
		new Main().run();
	}
	
	static int num(int i,int j,int len,int[][][] cnt,int H,int W,int target) {
		int sum=0;
		int INF=Integer.MAX_VALUE/3;
		for(int v=0;v<26;++v) {
			int p=i+len-1;
			int q=j+len-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]);
			if(a==INF) sum=INF;
			else if(a>0) ++sum;
			if(sum==target)break;
			else if(sum+(26-v-1)<target)break;
		}
		return sum;
	}
	
	static void run() {
		FastScanner sc=new FastScanner();
		int H=sc.nextInt();
		int W=sc.nextInt();
		int K=sc.nextInt();
		int[][][] cnt=new int[26][H+2][W+2];
		for(int i=1;i<=H;++i) {
			char[] map=sc.next().toCharArray();
			for(int v=0;v<26;++v) {
				for(int j=1;j<=W;++j) {
					cnt[v][i][j]=cnt[v][i][j-1]+cnt[v][i-1][j]-cnt[v][i-1][j-1];
					if((int)(map[j-1]-'a')==v) ++cnt[v][i][j];
				}
			}
		}
		long ans=0;
		int[][] lower=new int[2][W+1];
		int[][] upper=new int[2][W+1];
		for(int i=1;i<=H;++i) {
			for(int j=1;j<=W;++j) {
				int tmp=Math.max(Math.max(upper[(i-1)%2][j-1],upper[(i-1)%2][j])-1, 0);
				upper[i%2][j]=Math.max(upper[i%2][j-1]-1, tmp);
				tmp=Math.max(Math.max(lower[(i-1)%2][j-1],lower[(i-1)%2][j])-1, 0);
				lower[i%2][j]=Math.max(lower[i%2][j-1]-1, tmp);
				while(num(i,j,upper[i%2][j],cnt,H,W,K+1)<=K) {
					++upper[i%2][j];
				}
				while(num(i,j,lower[i%2][j],cnt,H,W,K)<K) {
					++lower[i%2][j];
				}
				ans+=upper[i%2][j]-lower[i%2][j];
			}
			
		}
		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());}
}