#include <bits/stdc++.h>
#define MAXn 1000010
using namespace std;

string s;
int n;
int l = 1, r;
int stk[MAXn], top;

int main()
{
    cin >> s;
    n = s.size();
    for(int i = 0; i < n; i++)
    {
        if(s[i] == '(' || s[i] == '[')
            stk[++top] = i;
        else if(s[i] == ')')
        {
            if(top == 0) continue;
            if(s[stk[top]] == '(')
            {
                if(i - stk[top] + 1 > r - l + 1)
                {
                    l = stk[top];
                    r = i;
                }
                top--;
            }
            else top = 0;
        }
        else 
        {
            if(top == 0) continue;
            if(s[stk[top]] == '[')
            {
                if(i - stk[top] + 1 > r - l + 1)
                {
                    l = stk[top];
                    r = i;
                }
                top--;
            }
            else top = 0;
        }
    }
    for(int i = l; i <= r; i++)
        cout << s[i];
    
    return 0;
}