#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

const long long MAX = 5100000;
const long long INF = 1LL << 60;
//const long long mod = 1000000007LL;
//const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

int dp[2007][2007][26];
string vs[2007];
int main()
{
	
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	
	ll H, W, K; cin >> H >> W >> K;
	for (ll i = 0; i < H; i++) {
		cin >> vs[i];
	}
	for (ll i = 0; i <= H; i++)for (ll j = 0; j <= W; j++)for (ll k = 0; k < 26; k++) dp[i][j][k] = (1 << 30);
	for (ll k = 0; k < 26; k++) {
		for (ll i = H - 1; i >= 0; i--) {
			for (ll j = W - 1; j >= 0; j--) {
				int f = min(H - i, W - j);
				if (vs[i][j] - 'a' == k) dp[i][j][k] = 1;
				else dp[i][j][k] = min({ f,dp[i + 1][j][k],dp[i][j + 1][k],dp[i + 1][j + 1][k] }) + 1;
			}
		}
	}
	ll res = 0;
	for (ll i = 0; i < H; i++) {
		for (ll j = 0; j < W; j++) {
			sort(dp[i][j], dp[i][j] + 26);
			if (K == 26) res += min(H - i, W - j) + 1 - dp[i][j][K - 1];
			else res += dp[i][j][K] - dp[i][j][K - 1];
		}
	}
	cout << res << endl;
	return 0;
}