#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>

using namespace std;

#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

// #define int short
#define endl "\n"

const long long INF = (long long)1e18;
const long long MOD = (long long)1e9 + 7; 

string yn(bool f){return f?"Yes":"No";}
string YN(bool f){return f?"YES":"NO";}

const int NUM = 26;
int H, W, K;
int con[2005][2005][26];
string squares[2000];

inline int check(int y, int x, int sz){
	sz--;
	
	if(sz < 0) return 0;
	
	int y2 = y + sz, x2 = x + sz;
	
	if(y2 >= H || x2 >= W) return NUM+1;
	
	int level = 0;
	
	for(int i = 0; i < NUM; i++){
		level += !!(con[y2+1][x2+1][i] - con[y2+1][x][i] - con[y][x2+1][i] + con[y][x][i]);
	}
	
	return level;
}

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	// cout<<fixed<<setprecision(10);
	
	long long ans = 0;
	
	
	cin>>H>>W>>K;
	
	for(int i = 0; i < H; i++){
		cin>>squares[i];
		for(int j = 0; j < W; j++){
			con[i+1][j+1][squares[i][j] - 'a']++;
		}
	}
	
	for(int k = 0; k < NUM; k++){
		for(int i = 0; i <= H; i++){
			for(int j = 0; j <= W; j++){
				con[i][j+1][k] += con[i][j][k];
				if(i) con[i][j][k] += con[i-1][j][k];
			}
		}
	}
	
	for(int i = 0; i < H; i++){
		for(int j = 0; j < W; j++){
			int minimum = (H-i < W-j)?H-i:W-j;
			
			int h = minimum + 1, l = 0, m, a;
			
			while(l + 1 < h){
				m = (l + h)/2;
				
				if(check(i,j,m) > K){
					h = m;
				} else {
					l = m;
				}
			}
			
			a = h;
			
			h = minimum + 1, l = 0;
			
			while(l + 1 < h){
				m = (l + h)/2;
				
				if(!(check(i,j,m) < K)){
					h = m;
				} else {
					l = m;
				}
			}
			
			ans += a - l - 1;
		}
	}
	
	cout<<ans<<endl;
	
	return 0;
}