#include <stdio.h>

int is_smaller(int K, char s[], char t[])
{
	int i;
	for (i = 0; i < K; i++) if (s[i] > t[i]) return 0;
	return 1;
}

int main()
{
	int i, N, K;
	char S[200001][10];
	scanf("%d %d", &N, &K);
	for (i = 1; i <= N; i++) scanf("%s", S[i]);
	
	int j, next[200003], prev[200003];
	for (i = 1, next[0] = 1, prev[N+1] = N; i <= N; i++) {
		next[i] = i + 1;
		prev[i] = i - 1;
	}
	for (i = 1; i <= N; i++) {
		for (j = next[0]; j < i; j = next[j]) {
			if (is_smaller(K, S[j], S[i])) {
				j = prev[j];
				next[j] = next[next[j]];
				prev[next[j]] = j;
			} else if (is_smaller(K, S[i], S[j])) {
				next[prev[i]] = next[i];
				prev[next[i]] = prev[i];
				break;
			}
		}
	}
	
	int ans = 0;
	for (i = next[0]; i <= N; i = next[i]) ans++;
	printf("%d\n", N - ans);
	fflush(stdout);
	return 0;
}