#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<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<tuple>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
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 Per(i,sta,n) for(int i=n-1;i>=sta;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 long double ld;
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;

int h,w,t;
string c[2010];
vector<vector<ll>> dp[26];

void solve(){
    cin >> h >> w >> t;
    rep(i,h){
        cin >> c[i];
    }
    int ord_a='a';
    rep(k,26){
        dp[k].resize(h+1,vector<ll>(w+1,INF));
        per(i,h){
            per(j,w){
                if(c[i][j]==ord_a+k) dp[k][i][j]=1;
                else dp[k][i][j]=min(INF,1+min(min(dp[k][i+1][j],dp[k][i][j+1]),dp[k][i+1][j+1]));
            }
        }
    }
    ll ans=0;
    rep(i,h){
        rep(j,w){
            vector<ll> s={INF};
            rep(k,26) {
                s.push_back(dp[k][i][j]);
                //cout << k << " " << i << " " << j << " " << dp[k][i][j] << endl;
            }
            sort(s.begin(),s.end());
            //rep(k,27) cout << k << " " << i << " " << j << " " << s[k] << endl;
            ll m=min(h-i,w-j);
            if (m>=s[t-1]) ans+=min(s[t],m+1)-s[t-1];
            //cout << ans << endl;
        }
    }

    cout << ans << endl;


}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(50);
    solve();
}