#include <fstream>
#include <algorithm>
#include <cstring>
using namespace std;
ifstream in("substr.in");
ofstream out("substr.out");
int n,k,r,maxi;
char s[16401],*f[16401],*a,*b;
bool cmp(char *a,char *b)
{ if(strcmp(a,b)<0)
     return 1;
  return 0;
}
int main()
{ in>>n>>k>>s;
  for(int i=0;i<n;i++)
    f[i]=s+i;
  sort(f,f+n,cmp);
  for(int i=0;i<=n-k;i++)
  { char *a,*b;
    a=f[i];
    b=f[i+k-1];
    r=0;
    while(*a==*b && strlen(a)>0 && strlen(b)>0)
        r++,a++,b++;
    maxi=max(maxi,r);
  }
  out<<maxi;
  in.close();
  out.close();
  return 0;
}