import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static string[] s_rd;
T RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
string RDR()() { return readln.chomp; }
T[] ARR(T = long)(in string str, T fix = 0) { auto r = str.split.to!(T[]); r[] += fix; return r; }
T[] RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[][] RDA2(T = long)(size_t n, T[] fix = []) { auto r = new T[][](n); foreach (i; 0..n) { r[i] = readln.chomp.split.to!(T[]); foreach (j, e; fix) r[i][j] += e; } return r; }
size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
long lcm(long x, long y) { return x * y / gcd(x, y); }

long mod = 10^^9 + 7;
//long mod = 998244353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

int binarySearch(alias pred, T)(in T[] arr, bool isLower = true)
{ 
	int ok, ng;
	if (isLower) { ok = cast(int)arr.length; ng = -1; }
	else		 { ok = -1; ng = cast(int)arr.length; }
	while (abs(ok-ng) > 1)
	{
		auto mid = (ok+ng) / 2;
		if (unaryFun!pred(arr[mid]))
			ok = mid;
		else ng = mid;
	}
	return ok;
}

void main()
{
	auto H = RD;
	auto W = RD;
	auto K = RD;
	auto c = new string[](H);
	foreach (i; 0..H)
	{
		c[i] = RD!string;
	}
	auto dp = new int[][][](H+1, W+1, 26);
	foreach (y; 0..H)
	{
		foreach (x; 0..W)
		{
			dp[y+1][x+1][] += dp[y+1][x][];
			dp[y+1][x+1][] += dp[y][x+1][];
			dp[y+1][x+1][] -= dp[y][x][];
			++dp[y+1][x+1][c[y][x]-'a']; 
		}
	}

	auto num = new long[](max(H, W));
	foreach (i; 0..num.length) num[i] = i+1;
	long ans;
	foreach (y; 0..H)
	{
		foreach (x; 0..W)
		{
			bool f(bool F)(long a)
			{
				auto d = dp[y+a][x+a].dup;
				d[] -= dp[y+a][x][];
				d[] -= dp[y][x+a][];
				d[] += dp[y][x][];
				long cnt;
				foreach (e; d)
				{
					if (e != 0) ++cnt;
				}
				auto ans = F ? cnt >= K : cnt <= K;
				//debug writeln(F, " ", ans, " ", cnt, " ", y+1, ":", x+1, " ", a, " ", d);
				return ans;
			}
			auto remain = min(H-y, W-x);
			auto l      = binarySearch!(f!true)(num[0..remain], true);
			auto r      = binarySearch!(f!false)(num[0..remain], false);
			auto cnt    = max(0, r - l + 1);
			ans        += cnt;
			debug writeln(y+1, ":", x+1, " ", cnt, " ", l, ":", r, " ", num[0..remain]);
		}
	}
	
	writeln(ans);
	stdout.flush();
	debug readln();
}