#include <bits/stdc++.h>
using namespace std;
const int N = int(1e5) + 10;
const int K = 11;
const int MOD = int(1e9) + 7;
const int INF = int(2e9) + 5;
const long long INF64 = 1e18;
int dp[N];
void solve() {
  string s;
  cin >> s;
  stack<int> st;
  int n = s.length();
  for (int i = 0; i < n; i++) {
    if (s[i] == '(' || s[i] == '[' || s[i] == '{') {
      st.push(i);
    } else {
      if (st.empty()) continue;
      int t = st.top();
      if (s[i] == ')' && s[t] == '(') {
        dp[i] = i - t + 1;
        if (t > 0) dp[i] += dp[t - 1];
        st.pop();
      } else if (s[i] == ']' && s[t] == '[') {
        dp[i] = i - t + 1;
        if (t > 0) dp[i] += dp[t - 1];
        st.pop();
      } else if (s[i] == '}' && s[t] == '{') {
        dp[i] = i - t + 1;
        if (t > 0) dp[i] += dp[t - 1];
        st.pop();
      } else {
        while (!st.empty()) st.pop();
      }
    }
  }
  int i = n - 1;
  int mx = 0, index = -1;
  while (i >= 0) {
    if (dp[i] == 0) {
      i--;
      continue;
    }
    int cnt = 0, t = i;
    int x = dp[i];
    for (int j = 0; j < x; j++) {
      if (s[i] == '[') cnt++;
      i--;
    }
    if (cnt > mx) {
      mx = cnt;
      index = t;
    }
  }
  if (mx == 0) {
    cout << 0 << '\n';
    return;
  }
  cout << mx << '\n';
  cout << s.substr(index + 1 - dp[index], dp[index]);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t;
  t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
