#include <bits/stdc++.h>
using namespace std;
int n, i, j, sk[200000], ct[200000], ans, u, le;
char ss[200000], c;
void ue(int a, int u1, int l) {
  if (a > ans) ans = a, u = u1, le = l;
}
int main() {
  scanf("%s", &ss);
  n = strlen(ss);
  for (int i = (0); i <= (n - 1); i++) {
    if (i) ct[i] = ct[i - 1];
    if (ss[i] == '[') ct[i]++;
  }
  sk[0] = 0;
  for (int i = (0); i <= (n - 1); i++) {
    c = ss[i];
    if (sk[0]) {
      if (c == ')' && ss[sk[sk[0]]] == '(' ||
          c == ']' && ss[sk[sk[0]]] == '[') {
        sk[0]--;
        if (sk[0])
          ue(ct[i] - ct[sk[sk[0]]], sk[sk[0]] + 1, i - sk[sk[0]]);
        else
          ue(ct[i], 0, i + 1);
      } else {
        sk[++sk[0]] = i;
      }
    } else {
      sk[++sk[0]] = i;
    }
  }
  printf("%d\n", ans);
  if (ans)
    for (int i = (u); i <= (u + le - 1); i++) printf("%c", ss[i]);
  cout << endl;
}
