#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define File(s) freopen(s".in","r",stdin),freopen(s".out","w",stdout)
#define maxn 500010

using namespace std;
typedef long long llg;

char a[maxn];
int l[maxn],s[maxn][26],f[maxn];
int head[maxn],next[maxn],to[maxn];
int tt,la,ans,dt;
bool w[maxn];

void add(int c,int n){
	int p=la;
	while(a[n-l[p]-1]!=a[n]) p=f[p];
	if(!s[p][c]){
		int np=++tt,k=f[p]; l[np]=l[p]+2;
		while(a[n-l[k]-1]!=a[n]) k=f[k];
		f[np]=s[k][c]; s[p][c]=np;
	}
	la=s[p][c];
}

void link(int x,int y){to[++dt]=y;next[dt]=head[x];head[x]=dt;}
void dfs(int u){
	if(~l[u]&1) w[l[u]]=1;
	if(l[u]%4==0 && w[l[u]>>1]) ans=max(ans,l[u]);
	for(int i=head[u];i;i=next[i]) dfs(to[i]);
	w[l[u]]=0;
}

int main(){
	File("a");
	l[++tt]=-1; f[0]=1;
	int n;scanf("%d %s",&n,a+1);
	for(int i=1;i<=n;i++) add(a[i]-‘a‘,i);
	for(int i=tt;i>1;i--) link(f[i],i);
	dfs(1); dfs(0); printf("%d",ans);
	return 0;
}