#include <iostream>
#include <vector>
#include <functional>
#include <queue>
using namespace std;
typedef long long ll;

int H,W,K;
vector<vector<char>> F(2001,vector<char>(2001));
vector<vector<vector<int>>> dp(26,vector<vector<int>>(2001,vector<int>(2001,-1)));

ll rec(int x,int y,int c){
    if(dp[c][x][y]!=-1) return dp[c][x][y];
    if(F[x][y]-'a'==c) return dp[c][x][y] = 1;
    ll res = min(H-x+1,W-y+1)+1;
    if(x!=H && y!=W) res = min(res,rec(x+1,y+1,c)+1);
    if(x!=H) res = min(res,rec(x+1,y,c)+1);
    if(y!=W) res = min(res,rec(x,y+1,c)+1);
    return dp[c][x][y] = res;
}


int main(){
    cin >> H >> W >> K;
    for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) cin >> F[i][j];
    ll ans = 0;
    for(int c=0;c<26;c++){
        for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) rec(i,j,c);
    }
    for(int i=1;i<=H;i++) for(int j=1;j<=W;j++){
        priority_queue<ll> Q;
        for(int c=0;c<26;c++){
            Q.push(dp[c][i][j]);
            if(Q.size()>K+1) Q.pop();
        }
        if(K==26) ans += min(H-i+1,W-j+1)+1-Q.top();
        else{
            ll r = Q.top(); Q.pop();
            ll l = Q.top();
            ans += r-l;
        }
    }
    cout << ans << endl;
}