#include <bits/stdc++.h>
int n, m;
int next[100010], stiv[100010], nr[100010];
char s[100010];
int main() {
  scanf("%s", s + 1);
  s[0] = ' ';
  n = strlen(s) - 1;
  for (int i = 1; i <= n; i++) {
    stiv[++m] = i;
    if (m > 1 && s[stiv[m]] == ')' && s[stiv[m - 1]] == '(') {
      next[stiv[m - 1]] = i;
      m -= 2;
    }
    if (m > 1 && s[stiv[m]] == ']' && s[stiv[m - 1]] == '[') {
      next[stiv[m - 1]] = i;
      m -= 2;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (s[i] == '[') nr[i]++;
    nr[i] += nr[i - 1];
  }
  int pmax = -1;
  for (int i = n; i > 0; i--)
    if (next[i]) {
      if (next[next[i] + 1] != 0) next[i] = next[next[i] + 1];
      if (pmax == -1 || nr[next[i]] - nr[i - 1] > nr[next[pmax]] - nr[pmax - 1])
        pmax = i;
    }
  if (pmax < 0)
    printf("0\n");
  else {
    printf("%d\n", nr[next[pmax]] - nr[pmax - 1]);
    for (int i = pmax; i <= next[pmax]; i++) printf("%c", s[i]);
    printf("\n");
  }
  return 0;
}
