B.cpp

/// by ahmed_drawy 20/12/19
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update
using namespace std;
using namespace __gnu_pbds;
template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
using namespace std;
#define ll long long
#define inN(arr ,  n) for(int i = 0 ; i< n ; ++i)  cin>>arr[i];
#define lp(i,start , end)       for(int i = start ; i<end ; ++i)
#define sz(v)                  (int)((v).size())
#define clr(v,d)                memset(v , d , sizeof(v));
#define pii pair<int, int>
#define vii vector<ll>
#define vi vector<int>
#define   endl '\n'
#define debug(x) cerr << x<<endl;
const double EPS = (1e-7);
void smile() {
    ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
#ifndef ONLINE_JUDGE
    freopen("/home/tw3/Desktop/Desktop/training/in.txt", "r", stdin);
  //  freopen("/home/tw3/Desktop/Desktop/training/out.txt" , "w" , stdout);
#endif
}
int dcmp(double x, double y) {	return fabs(x-y) <= EPS ? 0 : x < y ? -1 : 1;	}
int randomF() {
    return  (rand() * RAND_MAX + rand()) % 1000000;

}

ll MOD =(ll) 1e9 +7 ;
int mod(ll x){
    return (x%MOD + MOD)%MOD;
}
long long add(long long x, long long y)
{
    x += y;
    while(x >= MOD)
        x -= MOD;
    while(x < 0)
        x += MOD;
    return x;
}
ll multipy(ll x , ll y ){
    return ((x%MOD) *(y%MOD))%MOD;
}
long long power(long long base, int exp)
{
    if (!exp)
        return 1LL;
    ll sq = power(base, exp/2);
    sq = (sq * sq)%MOD;
    if (exp&1)
        sq = (sq * base)%MOD;
    return sq;
}
bool compareT(pair<int , int > & a , pair<int  ,int > &b){
    if(a.first != b.first)
        return a.first <b.first ;
    return a.second< b.second ;
}
double dist(int x , int y , int a , int b){
    return sqrt( (x-a)*(x-a) + (y-b)*(y-b));

}
bool isPalindrome (string s , int m ){
    for (int i = 0; i <m/2 ; ++i) {
        if(s[i] != s[m-i-1])
            return false ;
    }
    return true;

}
int main(){
    smile();


//    int t ;cin >> t ;
//    while(t--) {
        int n , m ; cin >> n >> m ;
        map <string ,pii> mmap ;

        for (int i = 0; i < n; ++i) {
            string s ;  cin >> s;
            string tp = s;
            reverse(tp.begin(), tp.end());
//            vector<>
            if(s!= tp ) {
                if (!mmap[s].first && !mmap[tp].first) {
                    mmap[s].first++;
                } else if (mmap[tp].first) {
                    mmap[tp].second++;
                } else if (mmap[s].first) {
                    mmap[s].first++;
                }
            }
            else {
                mmap[s].first++;
            }
        }
        int res = 0 ;
        string ss ; int mx = 0 ;
        string ans ;
        for(auto p : mmap ){
            if(p.second.first && p.second.second){
                res +=m*min(p.second.first , p.second.second)*2;
                ans +=p.first;

            }else if(p.second.first && isPalindrome(p.first , m)){

                if(p.second.first>mx ) {
                    mx = max(mx, p.second.first);
                    ss = p.first;
                }

            }
        }
        string tt = ans;
        reverse(tt.begin() , tt.end());
        for (int j = 0; j < mx; ++j) {
            ans+=ss;
        }

        cout << res + mx *m << endl;
        cout << ans << tt << endl;
//    }
}