#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int MAXN=5e4+5;
const ll inf=1e18;
struct Node {
	unordered_map <char,int> ch;
	int link,len;
}	tr[MAXN<<1];
int lst,tot;
inline void init() {
	for(int i=1;i<=tot;++i) {
		tr[i].link=tr[i].len=0;
		tr[i].ch.clear();
	}
	lst=tot=1;
}
inline int insert(char c) {
	int cur=++tot,p=lst;
	tr[cur].len=tr[lst].len+1;
	while(p&&!tr[p].ch.count(c)) tr[p].ch[c]=cur,p=tr[p].link;
	if(!p) tr[cur].link=1;
	else {
		int q=tr[p].ch[c];
		if(tr[q].len==tr[p].len+1) tr[cur].link=q;
		else {
			int r=++tot;
			tr[r]=tr[q],tr[r].len=tr[p].len+1;
			tr[cur].link=tr[q].link=r;
			while(p&&tr[p].ch[c]==q) tr[p].ch[c]=r,p=tr[p].link;
		}
	}
	return lst=cur;
} //SAM
char str[MAXN];
int q[MAXN],w[MAXN];
ll st[MAXN<<1],ed[MAXN<<1]; //ans[i]=qi+w
int buc[MAXN],rnk[MAXN<<1];
inline ll eval(int l,int r) { //get val(S[l,r])
	fill(st+1,st+tot+1,inf),fill(ed+1,ed+tot+1,-inf);
	init();
	fill(q+l,q+r+1,0),fill(w+l,w+r+1,0),fill(buc,buc+r-l+2,0);
	for(int i=l;i<=r;++i) {
		int p=insert(str[i]);
		st[p]=ed[p]=i;
	}
	for(int i=1;i<=tot;++i) ++buc[tr[i].len];
	for(int i=1;i<=r-l+1;++i) buc[i]+=buc[i-1];
	for(int i=1;i<=tot;++i) rnk[buc[tr[i].len]--]=i;
	for(int i=tot;i>1;--i) {
		int p=rnk[i];
		st[tr[p].link]=min(st[tr[p].link],st[p]);
		ed[tr[p].link]=max(ed[tr[p].link],ed[p]);
	}
	for(int i=2;i<=tot;++i) {
		auto add=[&](int x,int y,ll nq,ll nw) {
			if(x>y) return ;
			q[x]+=nq,w[x]+=nw;
			if(y<r) q[y+1]-=nq,w[y+1]-=nw;
		};
		int mx=tr[i].len,mn=tr[tr[i].link].len;
		add(st[i]+1,ed[i]-mx,0,mx-mn);
		add(max(st[i]+1,ed[i]-mx+1),ed[i]-mn,-1,ed[i]-mn+1);
	}
	ll ans=0;
	for(int i=l+1;i<=r;++i) {
		q[i]+=q[i-1],w[i]+=w[i-1];
		ans+=(q[i]*i+w[i])*(q[i]*i+w[i]);
	}
	return ans;
}
int n,k;
inline bool check(ll x) {
	int st=1,tot=0;
	while(st<=n) {
		if((++tot)>k) return false;
		auto judge=[&](int len) {
			if(st+len-1<=n) {
				return eval(st,st+len-1)<=x;
			}
			return false;
		};
		int k=1,len=0;
		while(judge(1<<k)) ++k;
		len=1<<(k-1);
		for(int d=k-2;~d;--d) if(judge(len+(1<<d))) len+=1<<d;
		st+=len;
	}
	return true;
}
signed main() {
//	freopen("word.in","r",stdin);
//	freopen("word.out","w",stdout);
	scanf("%d%d%s",&n,&k,str+1);
	ll l=0,r=inf,res=0;
	while(l<=r) {
		ll mid=(l+r)>>1;
		if(check(mid)) res=mid,r=mid-1;
		else l=mid+1;
//		cerr<<mid<<"\n";
	}
	printf("%lld\n",res);
	return 0;
}