#include <cstdlib>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
#define NMAX 17000
#define INF -1

typedef struct
{
	int half1, half2, val;
}
SUF;

using namespace std;

int size, cnt, k, log;
char text[NMAX], word[100];
SUF l[NMAX];
int arr[16][NMAX];

int sufCmp(const SUF& lVal, const SUF& rVal)
{
	return lVal.half1 != rVal.half1 ? lVal.half1 < rVal.half1 : lVal.half2 < rVal.half2;
}

void buildSuffixArray()
{
	for (int i = 0; i < size; ++i) arr[0][i] = text[i] <= 'Z' ? text[i] - 'A' : text[i] - 'a' + 26;

	for (int lg = 1, cnt = 1; lg < log; ++lg, cnt <<= 1)
	{
		for (int i = 0; i < size; ++i)
			l[i].half1 = arr[lg - 1][i], l[i].half2 = i + cnt < size ? arr[lg - 1][i + cnt] : INF, l[i].val = i;
		sort(l, l + size, sufCmp);
		for (int i = 0; i < size; ++i)
			arr[lg][l[i].val] = (i != 0 && !(l[i].half1 - l[i - 1].half1) && !(l[i].half2 - l[i - 1].half2)) ? arr[lg][l[i - 1].val] : i;
	}
}

int lcp(int i1, int i2)
{
    int len = 0;
	i1 = l[i1].val;
	i2 = l[i2].val;
	for (cnt = log; (i1 + (1 << cnt) >= size) && (i2 + (1 << cnt) >= size); --cnt);
    for (int ceva = 1 << cnt; cnt >= 0 && i1 < size && i2 < size; --cnt, ceva >>= 1)
		if (!(arr[cnt][i1] - arr[cnt][i2]))
            len += ceva, i1 += ceva, i2 += ceva;
    return len;
}

int main()
{
	FILE *fin = fopen("substr.in", "r"), *fout = freopen("substr.out", "w", stdout);
	fscanf(fin, "%d %d\n", &size, &k);
	
	if (k == 1)
	{
		printf("%d\n", size);
		return 0;
	}

	while (1 << log < size) ++log;
	for (int i = 0; i < size; ++i) fscanf(fin, "%s",text);

	buildSuffixArray();

    int best = 0;

    for (int i = 0; i < size - k; ++i)
        best = max(best, lcp(i, i + k - 1));

    printf("%d\n", best);

	return 0;
}