#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
#define op(x) ((x&1)?x+1:x-1)
#define odd(x) (x&1)
#define even(x) (!odd(x))
#define lc(x) (x<<1)
#define rc(x) (lc(x)|1)
#define lowbit(x) (x&-x)
#define mp(x,y) make_pair(x,y)
typedef long long ll;
typedef unsigned long long ull;
typedef double db;
using namespace std;
const int MAXN = 1e5+10,INF = 1e9;
void tomin(int& x,int y){x=min(x,y);}
void tomax(int& x,int y){x=max(x,y);}

namespace SAM{
	int len[MAXN],fa[MAXN],nxt[MAXN][26],mn[MAXN],mx[MAXN],tot,lst;
	vector<int>e[MAXN];

	void Init(){
		rep(i,0,tot){
			len[i]=fa[i]=0;
			memset(nxt[i],0,sizeof nxt[i]);
			e[i].clear();
		}
		tot=lst=0;

		fa[0]=-1;
	}	

	void ext(int c,int ed){
		int cur=++tot,p=lst;
		mn[cur]=mx[cur]=ed;
		len[cur]=len[p]+1;

		while(p!=-1 && !nxt[p][c])nxt[p][c]=cur,p=fa[p];
		if(p!=-1){
			int q=nxt[p][c];
			if(len[q]==len[p]+1)fa[cur]=q;
			else{
				int r=++tot;
				mn[r]=INF,mx[r]=-INF;
				len[r]=len[p]+1;
				fa[r]=fa[q],fa[q]=fa[cur]=r;
				rep(j,0,25)nxt[r][j]=nxt[q][j];

				while(p!=-1 && nxt[p][c]==q)nxt[p][c]=r,p=fa[p];
			}
		}

		lst=cur;
	}
	void dfs(int u){
		for(auto v:e[u]){
			dfs(v);
			tomin(mn[u],mn[v]);
			tomax(mx[u],mx[v]);
		}
		assert(mn[u] <= mx[u]);
	}
	void build(){
		rep(i,1,tot)e[fa[i]].push_back(i);
		dfs(0);
	}
};
using namespace SAM;

int n,k;
char s[MAXN],t[MAXN];

ll w[MAXN];

void add(int l,int r,int v){
	w[l]+=v;w[r+1]-=v;
}

ll calc(char* s,int n){
	Init();

	rep(i,1,n)ext(s[i]-'a',i);
	build();

	rep(i,1,n-1)w[i] = 0;

	rep(i,1,tot){
		int L = len[fa[i]]+1,R = len[i];
		R = min(R,mx[i]-mn[i]);
		if(L>R)continue;
		
		add(mn[i],mn[i],R-L+1);
		add(mx[i]-R+1,mx[i]-L+1,-1);
	}

	rep(i,1,n-1)w[i]+=w[i-1];
	rep(i,1,n-1)w[i]+=w[i-1];

	ll sum = 0;

	rep(i,1,n-1)sum += w[i]*w[i];
	return sum;
}
ll calc(int x,int y){
	rep(i,1,y-x+1)t[i] = s[x+i-1];
	return calc(t,y-x+1);
}

int chk(int p,int q,int d,ll mid){
	if(q+(1<<d) > n)return 0;

	return calc(p,q+(1<<d) ) <= mid;
}

int chk(ll mid){
	int cnt = 0,p = 1;
	while(p <= n){
		int q = p;
		rep(j,0,20)if(chk(p,q,j,mid)){
			q += (1<<j);
		}else{
			per(jj,j-1,0)if(chk(p,q,jj,mid)){
				q += (1<<jj);
			}
			break;
		}

		p=q+1;cnt++;
	}

	return cnt <= k;
}

int main(){
	//freopen("word.in","r",stdin);
	//freopen("word.out","w",stdout);

	cin>>n>>k>>(s+1);

	ll L = 0,R = 1e18,res = -1;
	while(L<=R){
		ll mid = (L+R)>>1;
		if(chk(mid)){
			res = mid;
			R = mid-1;
		}else{
			L = mid+1;
		}
	}

	assert(res != -1);
	cout<<res<<endl;

    return 0;
}