#include <bits/stdc++.h>
using namespace std;
stack<int>s;

int main() {
	string str;
	cin >> str;
	int maxlen = 0, start = 0;
	s.push(-1);
	for (int i = 0; i < str.size(); i++) {
		if (str[i] == '(' || str[i] == '[')
			s.push(i);
		else {
			if (s.size() == 1) {
				s.pop();
				s.push(i);
			} else {
				char top = str[s.top()];
				s.pop();
				if (str[i] == ')' && top == '(' || str[i] == ']' && top == '[') {
					int len = i - s.top();
					if (maxlen < len) {
						maxlen = len;
						start = s.top() + 1;
					}
				} else {
					s.pop();
					s.push(i);
				}
			}
		}
	}
	cout << str.substr(start, maxlen);
	return 0;
}