#include <cstdio>
#include <vector>
#define Nmax 17000
#define P1 10021
#define P2 17029
#define P3 13529
#define X 4303
#define Y 3657
#define Z 9857

using namespace std;

int N,K;
char a[Nmax];
vector <int> H1[P1+10],H2[P2+10],H3[P3+10];

inline void AdaugaHash1(int x)
{
    int i=x%P1;
    H1[i].push_back(x);
}

inline void AdaugaHash2(int x)
{
    int i=x%P2;
    H2[i].push_back(x);
}

inline void AdaugaHash3(int x)
{
    int i=x%P3;
    H3[i].push_back(x);
}

inline int CautaHash1(int x)
{
    int i=x%P1,cnt=0;
    vector<int>::iterator it;
    for(it=H1[i].begin();it!=H1[i].end();++it)
        if(*it==x)
            ++cnt;
    return cnt;
}

inline int CautaHash2(int x)
{
    int i=x%P2,cnt=0;
    vector<int>::iterator it;
    for(it=H2[i].begin();it!=H2[i].end();++it)
        if(*it==x)
            ++cnt;
    return cnt;
}

inline int CautaHash3(int x)
{
    int i=x%P3,cnt=0;
    vector<int>::iterator it;
    for(it=H3[i].begin();it!=H3[i].end();++it)
        if(*it==x)
            ++cnt;
    return cnt;
}

inline bool Ok(int L)
{
    int v1=0,v2=0,v3=0,aux1=1,aux2=1,aux3=1,i;
    /*for(i=0;i<P1;++i)
        H1[i].clear();
    for(i=0;i<P2;++i)
        H2[i].clear();
    for(i=0;i<P3;++i)
        H3[i].clear();*/
    for(i=1;i<L;++i)
    {
        aux1=(aux1*X)%P1;
        aux2=(aux2*Y)%P2;
        aux3=(aux3*Z)%P3;
    }
    for(i=1;i<=L;++i)
    {
        v1=(v1*X+a[i]-'0')%P1;
        v2=(v2*Y+a[i]-'0')%P2;
        v3=(v3*Z+a[i]-'0')%P3;
    }
    AdaugaHash1(v1); AdaugaHash2(v2); AdaugaHash3(v3);
    for(i=L+1;i<=N;++i)
    {
        v1=(((v1-((a[i-L]-'0')*aux1)%P1+P1)%P1)*X+a[i]-'0')%P1;
        v2=(((v2-((a[i-L]-'0')*aux2)%P2+P2)%P2)*Y+a[i]-'0')%P2;
        v3=(((v3-((a[i-L]-'0')*aux3)%P3+P3)%P3)*Z+a[i]-'0')%P3;
        AdaugaHash1(v1);
        AdaugaHash2(v2);
        AdaugaHash3(v3);
    }
    v1=v2=v3=0;
    for(i=1;i<=L;++i)
    {
        v1=(v1*X+a[i]-'0')%P1;
        v2=(v2*Y+a[i]-'0')%P2;
        v3=(v3*Z+a[i]-'0')%P3;
    }
    if(CautaHash1(v1)>=K && CautaHash2(v2)>=K && CautaHash3(v3)>=K)
        return true;
    for(i=L+1;i<=N;++i)
    {
        v1=((((v1-((a[i-L]-'0')*aux1)%P1+P1))%P1)*X+a[i]-'0')%P1;
        v2=((((v2-((a[i-L]-'0')*aux2)%P2+P2))%P2)*Y+a[i]-'0')%P2;
        v3=((((v3-((a[i-L]-'0')*aux3)%P3+P3))%P3)*Z+a[i]-'0')%P3;
        if(CautaHash1(v1)>=K && CautaHash2(v2)>=K && CautaHash3(v3)>=K)
            return true;
    }
    return false;
}

int main()
{
    int st,dr,mij,sol;
    freopen ("substr.in","r",stdin);
    freopen ("substr.out","w",stdout);
    scanf("%d%d%s", &N,&K,(a+1));
    st=1; dr=N;
    while(st<=dr)
    {
        mij=(st+dr)/2;
        if(Ok(mij))
        {
            sol=mij;
            st=mij+1;
        }
        else
            dr=mij-1;
    }
    printf("%d\n", sol);
    return 0;
}