#include <bits/stdc++.h>
#define int long long
#define IOS ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
using namespace std;
int H,W,K;
string s[2005];
int dp[2005][2005][26];
signed main(){
	//freopen("square.in","r",stdin);
	//freopen("square.out","w",stdout);
	IOS
	cin>>H>>W>>K;
	for(int i=0;i<H;i++){
		cin>>s[i];
	}
	for(int i=H;i--;){
		for(int j=W;j--;){
			for(int c=0;c<26;c++){
				dp[i][j][c]=min({i+1<H?dp[i+1][j][c]:H-i,j+1<W?dp[i][j+1][c]:W-j,i+1<H&&j+1<W?dp[i+1][j+1][c]:2000})+1;
			}
			dp[i][j][s[i][j]-'a']=1;
		}
	}
	int ans=0;
	for(int i=0;i<H;i++){
		for(int j=0;j<W;j++){
			sort(dp[i][j],dp[i][j]+26);
			ans+=(K<26?dp[i][j][K]:min(H-i,W-j)+1)-dp[i][j][K-1];
		}
	}
	cout<<ans;
	return 0;
}