#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = mod * mod;
typedef pair<int, int> P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef pair<ll, ll> LP;
typedef vector<ll> vec;
typedef vector<string> svec;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-8;

int num[2000][2000][26];
int calc(int xl, int yl, int xr, int yr) {
	//cout << "aaa " << xl << " " << yl << " " << xr << " " << yr << endl;
	int res = 0;
	rep(i, 26) {
		int sum = num[xr][yr][i];
		if (xl > 0)sum -= num[xl - 1][yr][i];
		if (yl > 0)sum -= num[xr][yl - 1][i];
		if (xl > 0 && yl > 0) {
			sum += num[xl - 1][yl - 1][i];
		}
		if (sum > 0)res++;
	}
	return res;
}

char mp[2000][2000];
void solve() {
	int h, w, k; cin >> h >> w >> k;
	rep(i, h) {
		rep(j, w) {
			cin >> mp[i][j];
		}
	}
	rep(i, h) {
		rep(j, w) {
			num[i][j][mp[i][j] - 'a']++;
			if (i > 0) {
				rep(k, 26) {
					num[i][j][k] += num[i - 1][j][k];
				}
			}
			if (j > 0) {
				rep(k, 26) {
					num[i][j][k] += num[i][j-1][k];
				}
			}
			if (i > 0 && j > 0) {
				rep(k, 26) {
					num[i][j][k] -= num[i - 1][j - 1][k];
				}
			}
		}
	}
	ll ans = 0;
	rep(i, h) {
		rep(j, w) {
			int mn = min(h - i, w - j);
			ans += mn;
		}
	}
	//cout << ans << endl;
	rep(i, h) {
		//k未満
		int loc = i;
		int mn = min(h, i + w);
		for (int j = i;j< mn; j++) {
			if (loc < j)loc++;
			while (loc < mn&&calc(j, j - i, loc, loc - i) < k) {
				loc++;
			}
			ans -= loc-j;
		}
		//k越え
		loc = mn-1;
		for (int j = mn - 1; j >= i; j--) {
			while (loc > j&&calc(j, j - i, loc, loc - i) > k) {
				loc--;
			}
			ans -= mn-1 - loc;
		}
	}
	//cout << ans << endl;
	rep1(i, w - 1) {
		int loc = i;
		int mn = min(w, h + i);
		for (int j = i; j < mn; j++) {
			if (loc < j)loc++;
			while (loc < mn&&calc(j - i, j, loc - i, loc) < k) {
				loc++;
			}
			ans -= loc - j;
		}
		//k越え
		loc = mn - 1;
		for (int j = mn - 1; j >= i; j--) {
			while (loc > j&&calc(j - i, j, loc - i, loc) > k) {
				loc--;
			}
			ans -= mn - 1 - loc;
		}
	}
	cout << ans << endl;
	
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(10);
	//init();
	solve();
	//cout << "finish" << endl;
	//stop
	return 0;
}