#include <cstdio>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
int H, W, c[26]; char S[2009][2009]; short xsum[26][2009][2009], ysum[26][2009][2009];
long long solve(int K) {
	long long ans = 0;
	for (int i = 0; i < H; ++i) {
		int sz = 0, cnt = 0;
		for (int j = 0; j < W; ++j) {
			while (cnt < K && i + sz < H && j + sz < W) {
				for (int k = 0; k < 26; ++k) {
					short delta = 0;
					delta += xsum[k][i + sz][j + sz + 1] - xsum[k][i + sz][j];
					delta += ysum[k][i + sz][j + sz] - ysum[k][i][j + sz];
					if (delta > 0 && c[k] == 0) ++cnt;
					c[k] += delta;
				}
				++sz;
			}
			if (sz != 0) {
				ans += sz - 1;
				if (cnt < K) ++ans;
			}
			if (sz > 0) {
				--sz;
				for (int k = 0; k < 26; ++k) {
					short delta = 0;
					delta += xsum[k][i + sz][j + sz + 1] - xsum[k][i + sz][j];
					delta += ysum[k][i + sz][j] - ysum[k][i][j];
					c[k] -= delta;
					if (delta > 0 && c[k] == 0) --cnt;
				}
			}
		}
	}
	return ans;
}
int main() {
	int K;
	scanf("%d %d %d", &H, &W, &K);
	for (int i = 0; i < H; ++i) scanf("%s", S[i]);
	for (int i = 0; i < H; ++i) {
		for (int j = 0; j < W; ++j) {
			++xsum[S[i][j] - 'a'][i][j + 1];
			++ysum[S[i][j] - 'a'][i + 1][j];
		}
	}
	for (int i = 0; i < 26; ++i) {
		for (int j = 0; j < H; ++j) {
			for (int k = 0; k < W; ++k) {
				xsum[i][j][k + 1] += xsum[i][j][k];
			}
		}
		for (int j = 0; j < W; ++j) {
			for (int k = 0; k < H; ++k) {
				ysum[i][k + 1][j] += ysum[i][k][j];
			}
		}
	}
	long long res1 = solve(K);
	long long res2 = solve(K + 1);
	printf("%lld\n", res2 - res1);
	return 0;
}