# include <fstream>
# include <string>
# include <vector>
# define DIM 16484
# define PRAG 10000000
#define MOD1 100007
#define MOD2 100021
using namespace std;
ifstream fin("substr.in");
ofstream fout("substr.out");
char ch[DIM];
int n,k,step,i,j,h1,h2,p=73,p1,p2,r1,r2,val;
vector<long long> Hash[MOD1];
int el(char ch){
    if(ch>='0'&&ch<='9')
        return 53+ch-'0';
    if(ch>='a'&&ch<='z')
        return ch-'a'+1;
    if(ch>='A'&&ch<='Z')
        return ch-'A'+27;
}
bool Val(int q){
    p1=p2=1;
    int nr;
    int ok=0;
    for(i=1;i<=q;i++){
        h1=(p*h1+el(ch[i]))%MOD1;
        h2=(p*h2+el(ch[i]))%MOD2;
        if(i!=1){
            p1*=p;
            p2*=p;
            p1%=MOD1;
            p2%=MOD2;
        }
    }
    val=(1LL*h1*PRAG+h2)%MOD1;
    Hash[val].push_back(1LL*h1*PRAG+h2);
    nr=0;
    for(int j=0;j<Hash[val].size();j++)
        if(Hash[val][j]==1LL*h1*PRAG+h2)
            nr++;
    if(nr>=k){
        h1=h2=0;
        p1=p2=1;
        return 1;
    }
    for(i=q+1;i<=n;i++){
        r1=p1*el(ch[i-q])%MOD1;
        r2=p2*el(ch[i-q])%MOD2;
        h1-=r1;
        if(h1<0)
            h1+=MOD1;
        h2-=r2;
        if(h2<0)
            h2+=MOD2;
        h1=(h1*p+el(ch[i]))%MOD1;
        h2=(h2*p+el(ch[i]))%MOD2;
        val=(1LL*h1*PRAG+h2)%MOD1;
        Hash[val].push_back(1LL*h1*PRAG+h2);
        nr=0;
        for(int j=0;j<Hash[val].size();j++)
            if(Hash[val][j]==1LL*h1*PRAG+h2)
                nr++;
        if(nr>=k){
            h1=h2=0;
            p1=p2=1;
            return 1;
        }
    }

    h1=h2=0;
    p1=p2=1;
    return 0;
}
int sol(){
    for(step=1;step<=n-k+1;step*=2);
    for(j=0;step;step/=2)
        if(j+step<=n-k+1&&Val(j+step))
            j+=step;
    return j;
}
int main () {
    fin>>n>>k>>ch+1;
    fout<<sol();
    return 0;
}