#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100005;
const int MOD = 1000000007;
vector<int> v;
string str;
void read() {
  cin >> str;
  str += "(";
}
void solve() {
  int n = str.size();
  for (int i = 0; i < n; i++) {
    if (str[i] == '(') v.push_back(i);
    if (str[i] == '[') v.push_back(i);
    if (str[i] == ')') {
      if (v.size() == 0 || str[v[v.size() - 1]] != '(')
        v.push_back(i);
      else
        v.pop_back();
    }
    if (str[i] == ']') {
      if (v.size() == 0 || str[v[v.size() - 1]] != '[')
        v.push_back(i);
      else
        v.pop_back();
    }
  }
  int m = 0, res = -1;
  for (int i = 0; i < v.size(); i++) {
    int fr = 0, to = 0;
    if (i == 0)
      fr = 0;
    else
      fr = v[i - 1] + 1;
    to = v[i];
    int cnt = 0;
    for (int j = fr; j < to; j++) cnt += (str[j] == '[');
    if (m <= cnt) {
      m = cnt;
      res = i;
    }
  }
  cout << m << '\n';
  if (res == -1)
    cout << '\n';
  else if (res == 0) {
    cout << str.substr(0, v[res]);
  } else {
    cout << str.substr(v[res - 1] + 1, v[res] - v[res - 1] - 1);
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  read();
  solve();
  return 0;
}
