/*

This Submission is to determine how many 120/240 min const. delivery point there are.

//info
120 req. steps <= 5

test_case 1,6 == 50 delivery point
		 2,3,4,5,7 == 80 delivery point
				8,9 == 90 delivery point
				10 > 100 delivery point
*/
#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 0.00000001
#define LONG_INF 1e18
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 1000000007
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)

unsigned long xor128() {
	static unsigned long x = time(NULL), y = 362436069, z = 521288629, w = 88675123;
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
long long h, w;
int cnt[2002][2002][26];
long long earn_point(int left_x, int left_y, int right_x, int right_y, int t) {
	long long ans = cnt[right_x][right_y][t];
	ans -= cnt[left_x - 1][right_y][t];
	ans -= cnt[right_x][left_y - 1][t];
	ans += cnt[left_x - 1][left_y - 1][t];
	return ans;
}
long long func(long long k) {
	long long ans = 0;
	for (int i = 1; i <= w; ++i) {
		long long now_x = 1;
		long long now_y = i;
		long long back = 0;
		for (int j = 0; now_y <= w && now_x <= h; ++j) {
			while (true) {
				int now_cnt = 0;
				back++;
				REP(t, 26) {
					if (earn_point(back + 1, back + i, now_x, now_y, t) >= 1) {
						now_cnt++;
					}
				}
				if (now_cnt < k) {
					back--;
					break;
				}
			}
			int now_cnt = 0;
			REP(t, 26) {
				if (earn_point(back+1, back + i, now_x, now_y, t) >= 1) {
					now_cnt++;
				}
			}
			if (now_cnt >= k) {
				ans += back + 1LL;
			}
			now_x++;
			now_y++;
		}
	}
	for (int i = 2; i <= h; ++i) {
		long long now_x = i;
		long long now_y = 1;
		long long back = 0;
		for (int j = 0; now_y <= w && now_x <= h; ++j) {
			while (true) {
				int now_cnt = 0;
				back++;
				REP(t, 26) {
					if (earn_point(back + i, back + 1, now_x, now_y, t) >= 1) {
						now_cnt++;
					}
				}
				if (now_cnt < k) {
					back--;
					break;
				}
			}
			int now_cnt = 0;
			REP(t, 26) {
				if (earn_point(back + i, back + 1, now_x, now_y, t) >= 1) {
					now_cnt++;
				}
			}
			if (now_cnt >= k) {
				ans += (back + 1LL);
			}
			now_x++;
			now_y++;
		}
	}
	return ans;
}
int main(){
	iostream::sync_with_stdio(false);
	long long k;
	cin >> h >> w >> k;
	for (int i = 1; i <= h;++i) {
		string a;
		cin >> a;
		for (int q = 1; q <= w; ++q) {
			REP(j, 26) {
				cnt[i][q][j] += cnt[i][q - 1][j];
			 }
			cnt[i][q][a[q - 1] - 'a']++;
		}
	}
	for (int i = 1; i <= h; ++i) {
		for (int q = 1; q <= w; ++q) {
			REP(j, 26) {
				cnt[i][q][j] += cnt[i - 1][q][j];
			}
		}
	}
	cout << func(k) - func(k+1) << endl;
	return 0;
}