#include <bits/stdc++.h>
#define Nmax 16390
using namespace std;
ifstream f("substr.in");
ofstream g("substr.out");
char s[Nmax];
char *v[Nmax];
inline bool cmp(char *x, char *y)
{
    return strcmp(x,y)<0;
}
int main()
{
    int i,n,k;
    f>>n>>k;
    f>>(s+1);
    for(i=1;i<=n;i++)
        v[i]=(s+i);
    sort(v+1,v+n+1,cmp);
    int ans=1,val;
    char *p1,*p2;
    for(i=1;i<=n-k+1;i++)
    {
        val=0;
        p1=v[i];
        p2=v[i+k-1];
        while(*p1 and *p2 and *p1==*p2)
        {
            ++p1;
            ++p2;
            ++val;
        }
        ans=max(ans,val);
    }
    g<<ans;

    return 0;
}