#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-9;
const int i32INF = 1 << 30;
const long long i64INF = 1LL << 60;
const double mINF = 1e100;
class item {
 public:
  item(int _from = 0, int _to = 0, int _v = 0) : from(_from), to(_to), v(_v) {}
  int from, to, v;
};
char s[100100];
int sk[100100];
int to[100100];
int getSk(int i) {
  if (i < 0) return 0;
  return sk[i];
}
int main() {
  gets(s);
  int n = strlen(s);
  for (int i = 0; i < n; i++)
    if (s[i] == '[') sk[i] = 1;
  for (int i = 1; i < n; i++) sk[i] += sk[i - 1];
  if (n == 1) {
    cout << 0 << endl;
    return 0;
  }
  stack<pair<char, int> > st;
  for (int i = 0; i < n; i++) {
    if (s[i] == ']') {
      if (!st.empty() && st.top().first == '[') {
        to[st.top().second] = i;
        st.pop();
      } else
        st = stack<pair<char, int> >();
    } else if (s[i] == ')') {
      if (!st.empty() && st.top().first == '(') {
        to[st.top().second] = i;
        st.pop();
      } else
        st = stack<pair<char, int> >();
    } else {
      st.push(make_pair(s[i], i));
    }
  }
  vector<item> v;
  for (int i = 0; i < n; i++)
    if (to[i]) v.push_back(item(i, to[i], getSk(to[i]) - getSk(i - 1)));
  if (v.empty()) {
    cout << 0 << endl;
    return 0;
  }
  vector<item> r(1, v[0]);
  for (int i = 1; i < v.size(); i++) {
    if (r.back().to == v[i].from - 1) {
      r.back().to = v[i].to;
      r.back().v += v[i].v;
    } else if (r.back().to < v[i].from - 1)
      r.push_back(v[i]);
  }
  item res(r[0]);
  for (int i = 0; i < r.size(); i++)
    if (r[i].v > res.v) res = r[i];
  s[res.to + 1] = 0;
  printf("%d\n%s\n", res.v, s + res.from);
  return 0;
}
