#include <iostream>
#include <algorithm>

using namespace std;

using ll = long long;

int h,w,k;
int a[2000][2000], r[2001][2001][27];

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

    char c;
    for(int i=0;i<h;++i) {
        for(int j=0;j<w;++j) {
            cin >> c;
            a[i][j] = int(c - 'a');
        }
    }
    fill_n(r[0][0], 2001*2001*27, 1);
    for(int i=h-1;i>=0;--i) {
        for(int j=w-1;j>=0;--j) {
            for(int ci=0;ci<27;++ci) {
                if(a[i][j] == ci) {
                    r[i][j][ci] = 1;
                } else {
                    r[i][j][ci] = min(min(h-i, w-j) + 1, min({r[i+1][j][ci], r[i][j+1][ci], r[i+1][j+1][ci]}) + 1);
                }
            }
        }
    }

    ll ans = 0;
    for(int i=0;i<h;++i) {
        for(int j=0;j<w;++j) {
            sort(r[i][j], r[i][j]+26);
            if(r[i][j][k-1] == 1<<30) continue;
            if(r[i][j][k] == 1<<30) {
                ans += min(h-i, w-j) + 1 - r[i][j][k-1];
            } else {
                ans += r[i][j][k] - r[i][j][k-1];
            }
        }
    }

    cout << ans << endl;
}