B

#include <array>
#include <algorithm>
#include <numeric>
#include <iostream>
#include <vector>
#include <map>
#include <stack>
#include <cassert>
#include <iomanip>

using namespace std;

#define int long long

const long long INF = 1e18 + 7;
const int MAXN = 1e2 + 100;
const int N = 1e5 + 10;

array<array<int, MAXN>, MAXN> dp;
string s;

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> s;
    int n = s.size();
    for (int i = 0; i < n; ++i) {
        dp[i][i] = 1;
    }
    for (int len = 1; len < n; ++len) {
        for (int l = 0; l + len < n; ++l) {
            int r = l + len;
            dp[l][r] = max({dp[l + 1][r], dp[l][r - 1], dp[l + 1][r - 1] + (s[l] == s[r] ? 2: 0)});
        }
    }
    cout << dp[0][n - 1] << '\n';
    string ans(n, '#');
    int l = 0, r = n - 1;
    while (l <= r) {
        if (l == r && dp[l][r] == 1) {
            ans[l] = s[l];
            ++l;
            continue;
        }
        if (s[l] == s[r]) {
            ans[l] = s[l];
            ans[r] = s[r];
            ++l;
            --r;
        } else {
            if (dp[l + 1][r] > dp[l][r - 1]) {
                ++l;
            } else {
                --r;
            }
        }
    }
    for (auto i: ans) {
        if (i != '#') {
            cout << i;
        }
    }
}