#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;

const int L=1000005;
char s[L];
int l,f[L],Ans,id;

int main()
{
	scanf("%s",s+1);
	l=strlen(s+1);
	for(int i=2;i<=l;++i)
		if(s[i]=='('||s[i]=='[') continue;
		else
			if((s[i]==')'&&s[i-f[i-1]-1]=='(')
			||(s[i]==']'&&s[i-f[i-1]-1]=='['))
			{
				f[i]=f[i-1]+2+f[i-f[i-1]-2];
				if(f[i]>Ans) Ans=f[i],id=i;
			}
	for(int i=id-Ans+1;i<=id;++i) printf("%c",s[i]);
	putchar('\n');
	return 0;
}