#include <bits/stdc++.h>
using namespace std;
int dp[100100], ans, pos, l, cnt[100100];
char s[100100];
vector<int> v;
int main() {
  scanf("%s", s);
  l = strlen(s);
  for (int i = 0; i <= l; i++) dp[i] = -1;
  v.reserve(100100);
  for (int i = 0; i < l; i++) {
    if (s[i] == '(' || s[i] == '[')
      v.push_back(i);
    else {
      if (v.empty()) continue;
      if ((s[v.back()] == '(' && s[i] == ')') ||
          (s[v.back()] == '[' && s[i] == ']')) {
        dp[v.back()] = i;
        dp[i] = v.back();
        v.pop_back();
      } else {
        v.clear();
      }
    }
  }
  cnt[0] = (s[0] == '[');
  for (int i = 1; i < l; i++) cnt[i] = cnt[i - 1] + (s[i] == '[');
  for (int i = 0; i < l; i++) {
    int x = 0, st = i;
    bool fl = 0;
    while (dp[i] != -1) {
      fl = 1;
      if (i == 0)
        x += cnt[dp[i]];
      else
        x += (cnt[dp[i]] - cnt[i - 1]);
      i = dp[i] + 1;
    }
    if (ans < x) {
      ans = x;
      pos = st;
    }
    if (fl == 1) i--;
  }
  printf("%d\n", ans);
  if (ans > 0) {
    while (dp[pos] != -1) {
      int fin = dp[pos];
      for (int i = pos; i <= fin; i++) printf("%c", s[i]);
      pos = fin + 1;
    }
    printf("\n");
  }
  return 0;
}
