#include<cstdio>
#include<vector>
#define mod1 102931
#define mod2 104729
#define base 101
using namespace std;
char s[17000];
vector<pair<int,int> > table[103000];
int n;
int convert(char c){
    if(c>='A'&&c<='Z')
        return c-'A';
    if(c>='a'&&c<='z')
        return c-'a'+26;
    return c-'0'+52;
}
int get_count(int hash1,int hash2){
    int dim=table[hash1].size(),i;
    for(i=0;i<dim;i++)
        if(table[hash1][i].first==hash2){
            table[hash1][i].second++;
            return table[hash1][i].second;
        }
    table[hash1].push_back(make_pair(hash2,1));
    return 1;
}
int get_max_count(int l){
    int i,hash1=0,hash2=0,pow1=1,pow2=1,maximum,cnt;
    maximum=-1;
    for(i=0;i<mod1;i++)
        table[i].clear();
    for(i=0;i<l;i++){
        hash1=(hash1*base+convert(s[i]))%mod1;
        hash2=(hash2*base+convert(s[i]))%mod2;
        if(i!=0){
            pow1=(pow1*base)%mod1;
            pow2=(pow2*base)%mod2;
        }
    }
    cnt=get_count(hash1,hash2);
    if(cnt>maximum)
        maximum=cnt;
    for(i=l;i<n;i++){
        hash1=((hash1-pow1*convert(s[i-l]))%mod1+mod1)%mod1;
        hash1=(hash1*base+convert(s[i]))%mod1;
        hash2=((hash2-pow2*convert(s[i-l]))%mod2+mod2)%mod2;
        hash2=(hash2*base+convert(s[i]))%mod2;
        cnt=get_count(hash1,hash2);
        if(cnt>maximum)
            maximum=cnt;
    }
    return maximum;
}
int main(){
    freopen("substr.in","r",stdin);
    freopen("substr.out","w",stdout);
    int k,l1,l2,m,answer=-1;
    scanf("%d%d\n%s",&n,&k,&s);
    l1=1;
    l2=n;
    while(l1<=l2){
        m=(l1+l2)/2;
        if(get_max_count(m)>=k){
            l1=m+1;
            if(m>answer)
                answer=m;
        }
        else
            l2=m-1;
    }
    printf("%d",answer);
    return 0;
}