#import<bits/stdc++.h>
using namespace std;ifstream f("substr.in");ofstream g("substr.out");int N,K,i,s;char cuv[1<<14],*p[1<<14];bool cmp(char* x,char* y){return strcmp(x,y)<0;}main(){f>>N>>K>>cuv;for(i=0;i<N;++i)p[i]=cuv+i;sort(p,p+N,cmp);for(i=0;i<=N-K;++i){int nr=0;char *a=p[i],*b=p[i+K-1];while(*a==*b&&*a&&*b)++nr,++a,++b;s=max(s,nr);}g<<s;}