#include <bits/stdc++.h>

using namespace std;

using ll = long long;

struct segtree {
    int H, W;
    vector<vector<int>> dat;
    segtree(){}
    segtree(vector<vector<int>> &f) {
        H = W = 1;
        while(H < (int)f.size()) H <<= 1;
        while(W < (int)f[0].size()) W <<= 1;
        dat.assign(2*H-1,vector<int>(2*W-1,0));
        init(f);
    }
    void init(vector<vector<int>> &f) {
        for (int i = 0; i < (int)f.size(); i++)
            for (int j = 0; j < (int)f[0].size(); j++)
                dat[i+H-1][j+W-1] = f[i][j];
        for (int i = 2*H-2; i > H-2; i--)
            for (int j = W-2; j >= 0; j--)
                dat[i][j] = (dat[i][2*j+1] | dat[i][2*j+2]);
        for (int i = H-2; i >= 0; i--)
            for (int j = 0; j < 2*W-1; j++)
                dat[i][j] = (dat[2*i+1][j] | dat[2*i+2][j]);
    }
    int minimum(int li, int lj, int ri, int rj) { return minimum_h(li,lj,ri,rj,0,H,0); }
    int minimum_h(int li, int lj, int ri, int rj, int si, int ti, int k) {
        if(ri <= si or ti <= li) return 0;
        if(li <= si and ti <= ri) return minimum_w(lj,rj,0,W,k,0);
        const int mi = (si+ti)/2;
        return (minimum_h(li,lj,ri,rj,si,mi,2*k+1) | minimum_h(li,lj,ri,rj,mi,ti,2*k+2));
    }
    int minimum_w(int lj, int rj, int sj, int tj, int i, int k) {
        if(rj <= sj or tj <= lj) return 0;
        if(lj <= sj and tj <= rj) return dat[i][k];
        const int mj = (sj+tj)/2;
        return (minimum_w(lj,rj,sj,mj,i,2*k+1) | minimum_w(lj,rj,mj,tj,i,2*k+2));
    }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int h, w, K;
    cin >> h >> w >> K;
    vector<string> c(h);
    vector< vector<int> > masks(h, vector<int>(w, 0));
    for (int i = 0; i < h; i++) {
        cin >> c[i];
        for (int j = 0; j < w; j++) {
            masks[i][j] = (1 << (c[i][j] - 'a'));
        }
    }

    segtree seg(masks);

    ll ans = 0;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            int sz = min(i + 1, j + 1);

            int ng = 0, ok = sz + 1;
            while (ok - ng > 1) {
                int mid = (ok + ng) / 2;
                int val = seg.minimum(i + 1 - mid, j + 1 - mid, i + 1, j + 1);
                if (__builtin_popcount(val) >= K) {
                    ok = mid;
                } else {
                    ng = mid;
                }
            }

            if (ok <= 0 || ok > sz) continue;

            int ng2 = sz + 1, ok2 = -1;
            while (ng2 - ok2 > 1) {
                int mid = (ok2 + ng2) / 2;
                int val = seg.minimum(i + 1 - mid, j + 1 - mid, i + 1, j + 1);
                if (__builtin_popcount(val) <= K) {
                    ok2 = mid;
                } else {
                    ng2 = mid;
                }
            }

            if (ok2 <= 0 || ok2 > sz || ok > ok2) continue;
            ans += ok2 - ok + 1;
            //cerr << i << " " << j << ": " << ok << " " << ok2 << endl;
        }
    }
    cout << ans << "\n";
    return 0;
}