package main

import (
	"bufio"
	"fmt"
	"math/bits"
	"os"
	"strconv"
)

func configure(scanner *bufio.Scanner) {
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 1000005), 1000005)
}
func getNextString(scanner *bufio.Scanner) string {
	scanned := scanner.Scan()
	if !scanned {
		panic("scan failed")
	}
	return scanner.Text()
}
func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}
func getNextInt64(scanner *bufio.Scanner) int64 {
	i, _ := strconv.ParseInt(getNextString(scanner), 10, 64)
	return i
}
func getNextFloat64(scanner *bufio.Scanner) float64 {
	i, _ := strconv.ParseFloat(getNextString(scanner), 64)
	return i
}
func main() {
	fp := os.Stdin
	wfp := os.Stdout
	extra := 0
	if os.Getenv("I") == "IronMan" {
		fp, _ = os.Open(os.Getenv("END_GAME"))
		extra = 100
	}
	scanner := bufio.NewScanner(fp)
	configure(scanner)
	writer := bufio.NewWriter(wfp)
	defer func() {
		r := recover()
		if r != nil {
			fmt.Fprintln(writer, r)
		}
		writer.Flush()
	}()
	solve(scanner, writer)
	for i := 0; i < extra; i++ {
		fmt.Fprintln(writer, "-----------------------------------")
		solve(scanner, writer)
	}
}
func solve(scanner *bufio.Scanner, writer *bufio.Writer) {
	h := getNextInt(scanner)
	w := getNextInt(scanner)
	k := getNextInt(scanner)
	sq := makeGrid(h, w)
	for y := 0; y < h; y++ {
		s := getNextString(scanner)
		for x := 0; x < w; x++ {
			sq[y][x].b[0] = sq[y][x].b[0] | uint32(1<<uint(s[x]-'a'))
		}
	}
	for i := 1; i < 16; i++ {
		for y := 0; y < h && y+1<<uint(i) <= h; y++ {
			for x := 0; x < w && x+1<<uint(i) <= w; x++ {
				sq[y][x].b[i] = sq[y][x].b[i] | sq[y][x].b[i-1]
				sq[y][x].b[i] = sq[y][x].b[i] | sq[y+1<<uint(i-1)][x].b[i-1]
				sq[y][x].b[i] = sq[y][x].b[i] | sq[y][x+1<<uint(i-1)].b[i-1]
				sq[y][x].b[i] = sq[y][x].b[i] | sq[y+1<<uint(i-1)][x+1<<uint(i-1)].b[i-1]
			}
		}
	}
	var ans int
	lowers := make([][][2]int, h)
	for i := 0; i < h; i++ {
		lowers[i] = make([][2]int, w)
	}
	for i := 0; i < w; i++ {
		for lw := 0; lw < 2; lw++ {
			ly := 0
			lx := i
			ry := 1
			rx := i + 1
			for ry <= h && rx <= w && ry-ly > 0 {
				lowers[ly][lx][lw] = w + 1
				c := types(ly, lx, ry-ly, sq)
				if c < k+lw {
					if ry == h || rx == w {
						lx++
						ly++
						continue
					}
					ry++
					rx++
					continue
				}
				lowers[ly][lx][lw] = rx
				lx++
				ly++
			}
		}
	}
	for i := 1; i < h; i++ {
		for lw := 0; lw < 2; lw++ {
			ly := i
			lx := 0
			ry := i + 1
			rx := 1
			for ry <= h && rx <= w && ry-ly > 0 {
				lowers[ly][lx][lw] = h + 1
				c := types(ly, lx, ry-ly, sq)
				if c < k+lw {
					if ry == h || rx == w {
						lx++
						ly++
						continue
					}
					ry++
					rx++
					continue
				}
				lowers[ly][lx][lw] = ry
				lx++
				ly++
			}
		}
	}
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			ans += lowers[y][x][1] - lowers[y][x][0]
		}
	}
	fmt.Fprintln(writer, ans)
}

func types(y, x, m int, sq [][]square) int {
	z := 32 - bits.LeadingZeros32(uint32(m))
	b := sq[y][x].b[z-1]
	b = b | sq[y+m-1<<uint(z-1)][x].b[z-1]
	b = b | sq[y][x+m-1<<uint(z-1)].b[z-1]
	b = b | sq[y+m-1<<uint(z-1)][x+m-1<<uint(z-1)].b[z-1]
	c := bits.OnesCount32(b)
	return c
}
func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

type square struct {
	b [16]uint32
}

func makeGrid(h, w int) [][]square {
	index := make([][]square, h, h)
	data := make([]square, h*w, h*w)
	for i := 0; i < h; i++ {
		index[i] = data[i*w : (i+1)*w]
	}
	return index
}