#include <bits/stdc++.h>
using namespace std;
bool sortbysec(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.second < b.second);
}
int main() {
  string s;
  stack<pair<char, int> > a;
  cin >> s;
  vector<pair<int, int> > vec;
  vector<int> rectangle;
  int numrect = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '(') {
      a.push({'(', i});
    }
    if (s[i] == '[') {
      a.push({'[', i});
    }
    if (s[i] == ')') {
      if (!a.empty() && a.top().first == '(') {
        pair<int, int> k = a.top();
        a.pop();
        vec.push_back({k.second, i});
      } else {
        while (!a.empty()) a.pop();
      }
    }
    if (s[i] == ']') {
      if (!a.empty() && a.top().first == '[') {
        pair<int, int> k = a.top();
        a.pop();
        vec.push_back({k.second, i});
        numrect++;
      } else {
        while (!a.empty()) a.pop();
      }
    }
    rectangle.push_back(numrect);
  }
  int l = 0, r = 0;
  sort(vec.begin(), vec.end());
  int n = vec.size();
  int flag = 0;
  vector<pair<int, int> > vec1;
  for (int i = 0; i < n; i++) {
    l = vec[i].first;
    r = vec[i].second;
    vec1.push_back({l, r});
    if (i == n - 1) break;
    while (vec[i + 1].first > l && vec[i + 1].second < r) {
      i++;
    }
  }
  sort(vec1.begin(), vec1.end(), sortbysec);
  cout << endl;
  n = vec1.size();
  for (int i = 0; i < n - 1; i++) {
    l = vec1[i].first;
    r = vec1[i].second;
    while (vec1[i].second + 1 == vec1[i + 1].first) {
      r = vec1[i + 1].second;
      i++;
    }
    if (l != vec1[i].first) vec1.push_back({l, r});
  }
  int p = 0, q = 0;
  int maxrect = 0;
  for (auto x : vec1) {
    if (rectangle[x.second] - rectangle[x.first] >= maxrect) {
      maxrect = rectangle[x.second] - rectangle[x.first];
      p = x.first;
      q = x.second;
    }
  }
  if (vec.size() == 0) {
    cout << 0;
    return 0;
  }
  cout << maxrect << endl;
  for (int i = p; i <= q; i++) {
    cout << s[i];
  }
}
