// <DATETIME>
#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define pii pair<int, int>
#define fi first
#define se second
#define rep(i, x, y) for (int i = (x); i <= (y); i ++ )
#define per(i, x, y) for (int i = (x); i >= (y); i -- )
const int N = 2000 + 5, M = 1e6 + 5;
const int inf = 1e9, mod = 998244353;
const ll INF = 1e18, RMX = 2324814;
mt19937 rd(time(0));
uniform_int_distribution<int> dist(0, RMX);

int n, m, k;
int lt[N][N], rt[N][N];
char a[N][N];
int pre[N][N][26];

int calc(int a, int b, int c, int d){
	int res = 0;
	rep(x, 0, 25){
		int cnt = pre[c][d][x] - pre[a - 1][d][x] - pre[c][b - 1][x] + pre[a - 1][b - 1][x];
		if (cnt) res ++ ;
	}
	return res;
}

signed main(){
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    
    cin >> n >> m >> k;
    rep(i, 1, n) rep(j, 1, m) cin >> a[i][j];
    rep(i, 1, n){
		rep(j, 1, m){
			rep(x, 0, 25) pre[i][j][x] = pre[i][j - 1][x] + pre[i - 1][j][x] - pre[i - 1][j - 1][x];
			pre[i][j][a[i][j] - 'a'] ++ ;
		}
	}
    
    ll awa = 0;
    rep(i, 1, n){
		rep(j, 1, m){
			lt[i][j] = max(0, lt[i - 1][j - 1] - 1);
			rt[i][j] = max(0, rt[i - 1][j - 1] - 1);
			
			while (i + lt[i][j] <= n && j + lt[i][j] <= m && calc(i, j, i + lt[i][j], j + lt[i][j]) < k) lt[i][j] ++ ;
			while (i + rt[i][j] <= n && j + rt[i][j] <= m && calc(i, j, i + rt[i][j], j + rt[i][j]) <= k) rt[i][j] ++ ;
			awa += rt[i][j] - lt[i][j];
		}
	}
	cout << awa << "\n";
}