#include <cstdio>
#include <cstring>

#define file_in "substr.in"
#define file_out "substr.out"

#define mod 666013
#define mod1 33
#define nmax 20100


int n,k;
char s[nmax];
int hash[mod+20];

void citire()
{
	freopen(file_in,"r",stdin);
	freopen(file_out,"w",stdout);
	
	scanf("%d %d\n", &n, &k);
	gets(s);
}



int verif(int l)
{
	memset(hash,0,sizeof(hash));
	int x=0,put=1,i;
	for (i=0;i<l;++i)
	{
		if (i>0)
			put*=mod1;
		if (put>=mod)
			put%=mod;
		x*=mod1;
		x+=s[i]-'a'+1;
		if (x>=mod)
			x%=mod;
	}
	hash[x]++;
	for (i=l;i<=n;++i)
	{
		x-=(put*(s[i-l]-'a'+1)%mod);
		if (x<0)
			x+=mod;
		if (x>=mod)
			x%=mod;
		x*=mod1;
		x+=s[i]-'a'+1;
		if (x>=mod)
			x%=mod;
		hash[x]++;
		if (hash[x]>=k)
			return 1;
	}
	return 0;
}

void solve()
{
	int i,step;
	for (step=1;step<=n;step<<=1);
	     for (i=0;step;step>>=1)
			 if (verif(i+step))
				 i+=step;
    printf("%d\n", i);
}

int main()
{
	citire();
	solve();
	
	fclose(stdin);
	fclose(stdout);
	
	return 0;
	
}