using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Security.Cryptography;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (h, w, k) = (c[0], c[1], c[2]);
        var s = SList(h);
        WriteLine(Sq(h, w, k, s));
    }
    static long Sq(int h, int w, int k, string[] s)
    {
        var poslist = new int[26, h, w];
        for (var m = 0; m < 26; ++m) for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j) poslist[m, i, j] = int.MaxValue;
        for (var i = 0; i < 26; ++i)
        {
            Calc(h, w, s, (char)(i + 'a'), poslist);
        }
        var ans = 0L;
        var arr = new int[26];
        for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j)
        {
            for (var m = 0; m < 26; ++m) arr[m] = poslist[m, i, j];
            Array.Sort(arr);
            if (arr[k - 1] == int.MaxValue) continue;
            var next = Math.Min(k < 26 ? arr[k] : int.MaxValue, Math.Min(h - i, w - j));
            ans += Math.Max(0, next - arr[k - 1]);
        }
        return ans;
    }
    static void Calc(int h, int w, string[] s, char c, int[,,] poslist)
    {
        var m = c - 'a';
        var xlist = new List<int>[w];
        var xidx = new int[w];
        for (var i = 0; i < w; ++i) xlist[i] = new List<int>();
        for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j) if (s[i][j] == c) xlist[j].Add(i);
        for (var i = 0; i < h; ++i)
        {
            var pos = 0;
            for (var j = 0; j < w; ++j)
            {
                while (pos < j || (pos < w && (xidx[pos] == xlist[pos].Count || xlist[pos][xidx[pos]] - i > pos - j)))
                    ++pos;
                poslist[m, i, j] = Math.Min(poslist[m, i, j], pos == w ? int.MaxValue : pos - j);
                if (xidx[j] < xlist[j].Count && xlist[j][xidx[j]] == i) ++xidx[j];
            }
        }
        var ylist = new List<int>[h];
        var yidx = new int[h];
        for (var i = 0; i < h; ++i) ylist[i] = new List<int>();
        for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j) if (s[i][j] == c) ylist[i].Add(j);
        for (var j = 0; j < w; ++j)
        {
            var pos = 0;
            for (var i = 0; i < h; ++i)
            {
                while (pos < i || (pos < h && (yidx[pos] == ylist[pos].Count || ylist[pos][yidx[pos]] - j > pos - i)))
                    ++pos;
                poslist[m, i, j] = Math.Min(poslist[m, i, j], pos == h ? int.MaxValue : pos - i);
                if (yidx[i] < ylist[i].Count && ylist[i][yidx[i]] == j) ++yidx[i];
            }
        }
    }
}