//Author: Kevin
#include<bits/stdc++.h>
//#pragma GCC optimize("O2")
using namespace std;
#define ll long long
#define ull unsigned ll
#define pb emplace_back
#define mp make_pair
#define ALL(x) (x).begin(),(x).end()
#define rALL(x) (x).rbegin(),(x).rend()
#define srt(x) sort(ALL(x))
#define rev(x) reverse(ALL(x))
#define rsrt(x) sort(rALL(x))
#define sz(x) (int)(x.size())
#define inf 0x3f3f3f3f
#define pii pair<int,int>
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
#define longer __int128_t
void die(string S){puts(S.c_str());exit(0);}
namespace SAM
{
	const int maxn=100100;
	int tr[maxn][26];
	int fail[maxn],len[maxn],p1[maxn],p2[maxn];
	int tot=1,last=1;
	vector<int> G[maxn];
	void clear()
	{
		for(int i=1;i<=tot;i++)
		{
			memset(tr[i],0,sizeof(tr[i]));
			G[i].clear();
			p1[i]=p2[i]=len[i]=fail[i]=0;
			p1[i]=inf;
		}
		tot=last=1;
	}
	void extend(int w,int x)
	{
		int p=last;
		int np=++tot;
		p1[np]=p2[np]=x;
		while(p&&!tr[p][w])
		{
			tr[p][w]=np;
			p=fail[p];
		}
		if(!p) fail[np]=1;
		else
		{
			int q=tr[p][w];
			if(len[p]+1==len[q])
				fail[np]=q;
			else
			{
				int nq=++tot;
				len[nq]=len[p]+1;
				memcpy(tr[nq],tr[q],sizeof(tr[q]));
				fail[nq]=fail[q];
				fail[q]=nq;
				fail[np]=nq;
				while(p&&tr[p][w]==q)
				{
					tr[p][w]=nq;
					p=fail[p];
				}
			}
		}
		last=np;
	}
	void build()
	{
		for(int i=2;i<=tot;i++)
			G[fail[i]].pb(i);
	}
	void dfs(int u)
	{
		for(auto v:G[u])
		{
			dfs(v);
			p1[u]=min(p1[u],p1[v]);
			p2[u]=max(p2[u],p2[v]);
		}
	}
}
ll calc(string S)
{
	SAM::clear();
	for(int i=0;i<sz(S);i++)
		SAM::extend(S[i]-'a',i);
	SAM::build();
	SAM::dfs(1);
	vector<ll> val(sz(S)+5);
	for(int i=2;i<=SAM::tot;i++)
	{
		int L=SAM::p1[i];
		int R=SAM::p2[i];
		int l=SAM::len[SAM::fail[i]]+1;
		int r=SAM::len[i];
		if(R-l<L) continue;
		r=min(r,R-L);
		val[L]+=r-l+1;
		val[L+1]-=r-l+1;
		val[R-r+1]--;
		val[R-l+2]++;
	}
	for(int i=1;i<sz(val);i++)
		val[i]+=val[i-1];
	for(int i=1;i<sz(val);i++)
		val[i]+=val[i-1];
	ll ans=0;
	for(int i=1;i<sz(S);i++)
		ans+=val[i-1]*val[i-1];
	return ans;
}
int n,k;
string s;
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin>>n>>k>>s;
	ll L=0,R=1e18;
	while(L<R)
	{
		ll mid=(L+R)/2;
		int cur=0;
		for(int r=0;r<k;r++)
		{
			if(cur==n) break;
			if(calc(s.substr(cur,1))>mid) break;
			int len=1;
			while(true)
			{
				if(cur+len>n) break;
				if(calc(s.substr(cur,len))>mid) break;
				len<<=1;
			}
			int l2=1,r2=min(len,n-cur);
			while(l2<r2)
			{
				int mid2=(l2+r2+1)/2;
				if(calc(s.substr(cur,mid2))<=mid)
					l2=mid2;
				else
					r2=mid2-1;
			}
			cur+=l2;
		}
		if(cur==n)
			R=mid;
		else
			L=mid+1;
	}
	cout<<L<<'\n';
	return 0;
}