#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 23;
const int MOD = 1e9 + 7;
const int MAXN = 1e5 + 100;
int stk[MAXN];
int match[MAXN];
int sum[MAXN];
int main() {
  ios_base::sync_with_stdio(0);
  string s;
  cin >> s;
  int len = s.length();
  memset(match, -1, sizeof(match));
  int top = 0;
  for (int i = 0; i < len; ++i) {
    char need = '#';
    if (s[i] == '(' || s[i] == '[') {
      stk[top] = i;
      ++top;
    } else if (s[i] == ')') {
      need = '(';
    } else {
      need = '[';
    }
    if (need == '#') continue;
    if (top > 0 && s[stk[top - 1]] == need) {
      --top;
      match[i] = stk[top];
    } else {
      top = 0;
    }
  }
  for (int i = 0; i < len; ++i) {
    if (match[i] <= 0) continue;
    if (match[match[i] - 1] == -1) continue;
    match[i] = match[match[i] - 1];
  }
  for (int i = 0; i < len; ++i) {
    sum[i + 1] = sum[i] + (s[i] == '[');
  }
  int ans = 0, st = 0, end = 0;
  for (int i = 0; i < len; ++i) {
    if (match[i] == -1) continue;
    int le = match[i];
    int cur = sum[i + 1] - sum[le];
    if (cur > ans) {
      ans = cur;
      st = le;
      end = i;
    }
  }
  cout << ans << "\n";
  if (ans > 0) {
    stringstream ss;
    for (int i = st; i <= end; ++i) {
      ss << s[i];
    }
    cout << ss.str() << "\n";
  }
  return 0;
}
