using namespace std;
#include<cstdio>
#include<algorithm>
#include<utility>
#define Nm (1<<15)
#define Lognm 16
char S[Nm];
int P[Lognm][Nm],n,k,stp,ans;
struct entry
{
    int p;
    pair<int,int> nr;
} L[Nm];

void read()
{
    freopen("substr.in","r",stdin);
    scanf("%d%d ",&n,&k);
    gets(S);
}

bool cmp(entry a, entry b)
{
    return a.nr<b.nr;
}

int lcp(int a, int b)
{
    int r=0,k;

    for(k=stp-1;k>=0 && a<n && b<n;--k)
        if(P[k][a]==P[k][b])
            a+=1<<k, b+=1<<k, r+=1<<k;
    return r;
}

void solve()
{
    int cnt,i;

    if(k==1)
    {
        ans=n;
        return;
    }

    for(i=0;i<n;++i)
        P[0][i]=S[i];
    for(stp=1,cnt=1;cnt<n;++stp,cnt<<=1)
    {
        for(i=0;i<n;++i)
        {
            L[i].p=i;
            L[i].nr=make_pair(P[stp-1][i],i+cnt<n?P[stp-1][i+cnt]:-1);
        }
        sort(L,L+n,cmp);
        P[stp][L[0].p]=0;
        for(i=1;i<n;++i)
            P[stp][L[i].p]=L[i].nr==L[i-1].nr?P[stp][L[i-1].p]:i;
    }

    for(i=0;i<=n-k;++i)
    {
        cnt=lcp(L[i].p,L[i+k-1].p);
        if(cnt>ans)
            ans=cnt;
    }
}

void write()
{
    freopen("substr.out","w",stdout);
    printf("%d\n",ans);
}

int main()
{
    read();
    solve();
    write();
    return 0;
}