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

unsigned int sq[2001][2001][11];

int level(int y, int x, int m){
    int z = 32-__builtin_clz((unsigned int)m);
    int zz = 1<<(z-1);
    return __builtin_popcount(sq[y][x][z-1]|sq[y+m-zz][x][z-1]|sq[y][x+m-zz][z-1]|sq[y+m-zz][x+m-zz][z-1]);
}
long long rangek(int y, int x, int h, int w, int k){
    long long ret = 0ll;
    int mx = min(h-y, w-x);
    long long lowers[2001][2];
    for(int i = 0; i < mx; i++){
        lowers[i][0] = 0;
        lowers[i][1] = 0;
    }
    for(int i = 0; i < 2; i++){
        int l = 0;
        for(int r = 1; r < mx + 1; r++){
            for(;r>l;){
                if(level(y+l,x+l,r-l) < k + i){
                    break;
                }
                l++;
            }
            lowers[r-1][i] = l;
            ret += lowers[r-1][0] - lowers[r-1][i];
        }
    }
    return ret;
}
int main() {
    int h,w,k;
    cin>>h>>w>>k;
    string s;
    for(int y = 0; y < h; y++){
        cin >> s;
        for(int x = 0; x < w; x++){
            sq[y][x][0] = 1<<(s[x]-'a');
        }
    }
    int z;
    for(int i = 1; i < 11;i++){
        z = 1<<(i-1);
        for(int y = 0; y < h&&y+(1<<i)<=h; y++){
            for(int x = 0; x < w&&x+(1<<i)<=w; x++){
                sq[y][x][i] = sq[y][x][i-1]|sq[y+z][x][i-1]|sq[y][x+z][i-1]|sq[y+z][x+z][i-1];
            }
        }
    }
    long long ans = 0ll;
    for(int x = 0; x < w; x++){
        ans += rangek(0, x, h, w, k);
    }
    for(int y = 1; y < h; y++){
        ans += rangek(y, 0, h, w, k);
    }
    cout<<ans<<endl;
}