#include<stdio.h>
#include<string.h>
#include<algorithm>
struct entry
{
	int x,y,p;
};
using namespace std;
int n,step,d;
char s[16390];
int p[16][16390];
entry l[16390];

bool cmp(entry a, entry b)
{
	return a.x==b.x?(a.y<=b.y):(a.x<=b.x);
}

int lcp(int x, int y)
{
	int k,ret=0;
	if(x==y)
		return n-x;
	for(k=step-1;k>=0 && x<n && y<n;--k)
		if(p[k][x]==p[k][y])
		{
			x+=1<<k;
			y+=1<<k;
			ret+=1<<k;
		}
	return ret;
}

int main()
{
	freopen("substr.in","r",stdin);
	freopen("substr.out","w",stdout);
	scanf("%d%d\n",&n,&d);
	gets(s);
	int i,j,step;
	for(i=0;i<n;i++)
		p[0][i]=s[i]-'a';
	for(j=1,step=1;(j<<1)<n;j=j<<1,step++)
	{
		for(i=0;i<n;i++)
		{
			l[i].x=p[step-1][i];
			l[i].y=p[step-1][i+j];
			l[i].p=i;
		}
		sort(l,l+n,cmp);
		p[step][l[0].p]=0;
		for(i=1;i<n;i++)
			p[step][l[i].p]=(l[i].x==l[i-1].x && l[i].y==l[i-1].y)?(p[step][l[i-1].p]):i;
	}
	int sol=0,x;
	for(i=0;i<=n-d;i++)
	{
		x=lcp(l[i].p,l[i+d-1].p);
		sol=sol>x?sol:x;
	}
	printf("%d\n",sol);
	return 0;
}