#include <bits/stdc++.h>
using namespace std;
int main()
{
int n, m;
cin >> n >> m;
string s[105], ans = "", rev = "";
for (int i = 0;i < n;i++)
cin >> s[i];
for (int i = 0; i < n;i++)
{
for (int j = i + 1;j < n;j++) {
string x = s[i];
reverse(x.begin(), x.end());
if (s[j] == x)
{
ans += s[i];
break;
}
}
}
for (int i = 0; i < n; i++)
{
bool fla = true;
for (int l = 0, k = s[i].size() - 1;l < k;l++, k--)
{
if (s[i][l] != s[i][k])
{
fla = false;
break;
}
}
if (fla) {
rev= s[i];
break;
}
}
cout << ans.size() * 2 + rev.size()<<"\n";
cout << ans << rev;
reverse(ans.begin(), ans.end());
cout << ans;
return 0;
}
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