#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>

using namespace std;

typedef long long LL;

const int N = 2010, INF = 0x3f3f3f3f;

LL ans;
int h, w, kk, a[N][N], f[2][N][30], num[N];

int main() {
    // freopen("square.in", "r", stdin);
    // freopen("square.out", "w", stdout);

    scanf("%d%d%d", &h, &w, &kk);
    for (int i = 1; i <= h; ++i) {
        string s;
        cin >> s;
        for (int j = 1; j <= w; ++j) {
            a[i][j] = s[j - 1] - 'a';
        }
    }

    memset(f, 0x3f, sizeof(f));
    for (int i = h; i >= 1; --i) {
        for (int j = w; j >= 1; --j) {
            int cnt = 0;
            for (int k = 0; k < 26; ++k) {
                f[i & 1][j][k] = INF;
                if (a[i][j] == k) f[i & 1][j][k] = 1;
                f[i & 1][j][k] = min(f[i & 1][j][k], min(f[i & 1 ^ 1][j][k], min(f[i & 1 ^ 1][j + 1][k], f[i & 1][j + 1][k])) + 1);
                num[++cnt] = f[i & 1][j][k];
            }
            nth_element(num + 1, num + kk, num + cnt + 1);
            int kth_e = num[kk];
            int len = min(h - i + 1, w - j + 1);
            int up = len + 1;
            if (kk < 26) {
                nth_element(num + 1, num + kk + 1, num + cnt + 1);
                up = min(up, num[kk + 1]);
            }
            if (kth_e <= len) ans += up - kth_e;
        }
    }

    printf("%lld\n", ans);

    return 0;
}