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

int rui[26][2010][2010] = {};
int h, w;
char c[2010][2010];

#define int long long

bool isOK(int i, int j, int ha, int num){

    int ans = 0;

    for(int x = 0;x < 26;x++){
        if(rui[x][i+ha+1][j+ha+1] - rui[x][i+ha+1][j] - rui[x][i][j+ha+1] + rui[x][i][j] > 0){
            ans++;
        }
    }

    return ans >= num;
}

signed main(){
    int k;
    cin >> h >> w >> k;

    for(int i = 0;i < h;i++){
        cin >> c[i];
        for(int j = 0;j < w;j++){
            rui[c[i][j]-'a'][i+1][j+1] = 1;
        }
    }

    for(int x = 0;x < 26;x++){
        for(int i = 0;i <= h;i++){
            for(int j = 0;j <= w;j++){
                rui[x][i+1][j] += rui[x][i][j];
            }
        }
    }

    for(int x = 0;x < 26;x++){
        for(int i = 0;i <= h;i++){
            for(int j = 0;j <= w;j++){
                rui[x][i][j+1] += rui[x][i][j];
            }
        }
    }

    int ans = 0;
    for(int i = 0;i < h;i++){
        for(int j = 0;j < w;j++){
            int l, r;
            l = 0;
            r = min(h-i, w-j);

            while(r-l > 1){
                int mid = (l+r)/2;
                if(isOK(i, j, mid, k)) r = mid;
                else l = mid;
            }

            int l2, r2;
            l2 = 0;
            r2 = min(h-i, w-j);
            while(r2-l2 > 1){
                int mid = (l2+r2)/2;
                if(isOK(i, j, mid, k+1)) r2 = mid;
                else l2 = mid;

            }

            ans += l2-l;
        }
    }

    cout << ans << endl;
    return 0;
}