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

//template
#define rep(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<typename T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<typename T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
//end

char s[2010][2010];
int dp[2010][2010][27];

int main(){
   int h,w,k; cin>>h>>w>>k;
   rep(i,0,h)cin>>s[i];
   for(int i=h;i>=0;i--)for(int j=w;j>=0;j--){
      rep(c,0,27){
         dp[i][j][c]=min({min(h-i,w-j)+1,dp[i][j+1][c],
            dp[i+1][j][c],dp[i+1][j+1][c]})+1;
      }
      if(islower(s[i][j]))dp[i][j][s[i][j]-'a']=1;
   }
   ll res=0;
   rep(i,0,h)rep(j,0,w){
      sort(dp[i][j],dp[i][j]+27);
      res+=dp[i][j][k]-dp[i][j][k-1];
   }
   cout<<res<<endl;
   return 0;
}