#include <iostream>
#include <vector>
#include <algorithm>
#include <array>
#include <iterator>
#include <string>
#include <cctype>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <cmath>
#include <ctime>
#include <iomanip>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <bitset>
#include <random>
#include <utility>
#include <functional>
using namespace std;
void Main()
{
	int H,W,K;
	cin >> H >> W >> K;
	int L = 1;
	while((1 << L) < min(H,W))
	{
		L++;
	}
	vector<vector<vector<int>>> dp(H,vector<vector<int>>(W,vector<int>(L)));
	for(int i = 0;i < H;i++)
	{
		string S;
		cin >> S;
		for(int j = 0;j < W;j++)
		{
			dp[i][j][0] = 1 << (S[j] - 'a');
		}
	}
	for(int k = 1;k < L;k++)
	{
		int t = 1 << (k - 1);
		for(int i = 0;i + 2 * t <= H;i++)
		{
			for(int j = 0;j + 2 * t <= W;j++)
			{
				dp[i][j][k] = dp[i][j][k - 1] | dp[i + t][j][k - 1] | dp[i][j + t][k - 1] | dp[i + t][j + t][k - 1];
			}
		}
	}
	vector<int> A(min(H,W) + 1);
	for(int i = 1;i < (int)A.size();i++)
	{
		while(1 << (A[i] + 1) <= i)
		{
			A[i]++;
		}
	}
	long long ans = 0;
	for(int i = 0;i < H;i++)
	{
		for(int j = 0;j < W;j++)
		{
			int lo = 0,up = 0;
			{
				int ng = 0,ok = min(H - i,W - j) + 1;
				while(ok - ng > 1)
				{
					int mid = (ok + ng) / 2;
					int a = A[mid];
					int cur = 0;
					int ni = i + mid - (1 << a);
					int nj = j + mid - (1 << a);
					cur |= dp[i][j][a];
					if(__builtin_popcount(cur) >= K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[ni][j][a];
					if(__builtin_popcount(cur) >= K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[i][nj][a];
					if(__builtin_popcount(cur) >= K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[ni][nj][a];
					if(__builtin_popcount(cur) >= K)
					{
						ok = mid;
						continue;
					}
					ng = mid;
				}
				lo = ok;
			}
			{
				int ng = 0,ok = min(H - i,W - j) + 1;
				while(ok - ng > 1)
				{
					int mid = (ok + ng) / 2;
					int a = A[mid];
					int cur = 0;
					int ni = i + mid - (1 << a);
					int nj = j + mid - (1 << a);
					cur |= dp[i][j][a];
					if(__builtin_popcount(cur) > K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[ni][j][a];
					if(__builtin_popcount(cur) > K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[i][nj][a];
					if(__builtin_popcount(cur) > K)
					{
						ok = mid;
						continue;
					}
					cur |= dp[ni][nj][a];
					if(__builtin_popcount(cur) > K)
					{
						ok = mid;
						continue;
					}
					ng = mid;
					/* cur |= dp[i][j][a]; */
					/* cur |= dp[ni][j][a]; */
					/* cur |= dp[i][nj][a]; */
					/* cur |= dp[ni][nj][a]; */
					/* if(__builtin_popcount(cur) > K) */
					/* { */
					/* 	ok = mid; */
					/* } */
					/* else */
					/* { */
					/* 	ng = mid; */
					/* } */
				}
				up = ok;
			}
			ans += up - lo;
		}
	}
	cout << ans << "\n";
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	/* cin >> tt; */
	while(tt--) Main();
}