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

using namespace std;

// #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;

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

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(10);
	
	long long ans = 0;
	
	vector<string> squares;
	vector<vector<vector<short>>> con;
	
	cin>>H>>W>>K;
	
	
	con.resize(H+2, vector<vector<short>>(W+2, vector<short>(NUM, 0)));
	squares.resize(H);
	
	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 h = min(H-i, W-j) + 1, l = 0, m;
			int a, b;
			
			while(l + 1 < h){
				m = (l + h)/2;
				
				if(check(con,i,j,m) > K){
					h = m;
				} else {
					l = m;
				}
			}
			
			a = h;
			
			h = min(H-i, W-j) + 1, l = 0;
			
			while(l + 1 < h){
				m = (l + h)/2;
				
				if(!(check(con,i,j,m) < K)){
					h = m;
				} else {
					l = m;
				}
			}
			
			b = l;
			
			ans += a - b - 1;
		}
	}
	
	cout<<ans<<endl;
	
	return 0;
}