$n^2+$ 特判过了？！

#include <bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define rll register ll
#define cll const ll
#define N 1000005
#define P 131
using namespace std;
inline ll read()
{
    rll x=0;bool f=1;register char c=getchar();
    while(c<48||c>57){if(c=='-') f=0;c=getchar();}
    while(c>=48&&c<=57){x=x*10+(c^48);c=getchar();}
    return f?x:-x;
}
inline void write(ll x)
{
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+48);
}
cll n=read();
ll ans,idt,last,len[N],mps[N],t[N][26];
char s[N];
bool check=1;
ull h[N],p[N];
map<ll,bool> flag;
inline ull hs(ll l,ll r){return h[r]-h[l-1]*p[r-l+1];}
inline ll get(ll pos,ll p)
{
	while(s[pos-1-len[p]]!=s[pos])
		p=mps[p];
	return p;
}
inline void insert(ll pos,ll x)
{
	cll p=get(pos,last);
	if(!t[p][x])
	{
		mps[++idt]=t[get(pos,mps[p])][x];
		t[p][x]=idt;
		len[idt]=len[p]+2;
	}
	last=t[p][x];
	rll q=last;
	while(q>1)
	{
		if((len[q]&1)&&flag[hs(pos-len[q]+1,pos)])
			ans=max(ans,(len[q]+1)/2);
		flag[hs(pos-len[q]+1,pos)]=1;
		q=mps[q];
	}
}
int main()
{
	scanf("%s",s+1);
	for(rll i=2;i<=n;i++)
		if(s[i]!=s[i-1])
			check=0;
	if(check)
	{
		write(n/2);
		return 0;
	}
	idt=1,len[1]=-1,mps[0]=1;
	p[0]=1;
	for(rll i=1;i<=n;i++)
	{
		p[i]=p[i-1]*P;
		h[i]=h[i-1]*P+s[i];
		insert(i,s[i]-'a');
	}
	write(ans);
	return 0;
}