#include <bits/stdc++.h>
using namespace std; typedef long double ld; typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define MP make_pair
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORR(x,arr) for(auto& x:arr)
#define PII pair<int, int>
#define FI first 
#define SE second
#define ALL(x) (x).begin(), (x).end()
const int INF=1<<30; const ll LINF=1LL<<60; const ll mod=1e9+7; const int NIL = -1;
template<class T>inline bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
//-------------------
int h,w,k;
char table[2000+5][2000+5];
int dp[2000+5][2000+5][27];
int base = int('a');

int main(){
	cin >> h >> w >> k;
	ll ans = 0;
	//memset(dp,-1,sizeof(dp));
	FOR(i,1,h) FOR(j,1,w){
		cin >> table[i][j];
	}
	for(int a=0; a<26;a++){
		for(int i=h+1; i>=1; i--){
			for(int j=w+1; j>=1; j--){
				if(table[i][j] == char(base+a)){
					dp[i][j][a] = 1; continue;
				}
				dp[i][j][a] = min(dp[i+1][j+1][a], min(dp[i+1][j][a],dp[i][j+1][a]))+1;
				dp[i][j][a] = min(dp[i][j][a], min(h+1-i, w+1-j)+2);
			}
		}
	}
	FOR(i,1,h){
		FOR(j,1,w){
			sort(dp[i][j], dp[i][j]+26);
			ans += 1LL * (dp[i][j][k] - dp[i][j][k-1]);
		}
	}
	cout << ans << endl;
    return 0;
}