#include <iostream>
#include <vector>
using namespace std;
typedef long long ll;
vector<vector<char>> F(2001,vector<char>(2001));
vector<vector<vector<int>>> sum(26,vector<vector<int>>(2001,vector<int>(2001,0)));
int calc(int x1,int y1,int x2,int y2){
    int res = 0;
    for(int c=0;c<26;c++) res += sum[c][x2][y2]-sum[c][x1-1][y2]-sum[c][x2][y1-1]+sum[c][x1-1][y1-1]>=1;
    return res;
}

int main(){
    int H,W,K;
    cin >> H >> W >> K;
    for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) cin >> F[i][j];
    for(char c='a';c<='z';c++){
        for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) if(F[i][j]==c){
            sum[c-'a'][i][j]++;            
        }
        for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) sum[c-'a'][i][j] += sum[c-'a'][i][j-1];
        for(int i=1;i<=H;i++) for(int j=1;j<=W;j++) sum[c-'a'][i][j] += sum[c-'a'][i-1][j];
    }
    ll ans = 0;
    for(int i=1;i<=H;i++) for(int j=1;j<=W;j++){
        ll l1 = 1,r1 = min(H-i+1,W-j+1)+1;
        while(l1+1<r1){
            ll m = (l1+r1)/2;
            int cnt = 0;
            if(calc(i,j,i+m-1,j+m-1)<=K) l1 = m;
            else r1 = m;
        }
        ll l2 = 0,r2 = l1+1;
        while(l2+1<r2){
            ll m = (l2+r2)/2;
            int cnt = 0;
            if(calc(i,j,i+m-1,j+m-1)<K) l2 = m;
            else r2 = m;
        }
        ans += l1-l2;
    }
    cout << ans << endl;
}