#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std; using namespace placeholders;

using LL = long long;
using ULL = unsigned long long;
using VI = vector< int >;
using VVI = vector< vector< int > >;
using VS = vector< string >;
using SS = stringstream;
using PII = pair< int, int >;
using VPII = vector< pair< int, int > >;
template < typename T = int > using VT = vector< T >;
template < typename T = int > using VVT = vector< vector< T > >;
template < typename T = int > using LIM = numeric_limits< T >;

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }
template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; };
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); };

#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &e : c )
#define ALL( c ) ( c ).begin(), ( c ).end()
#define AALL( a, t ) ( t* )a, ( t* )a + sizeof( a ) / sizeof( t )
#define DRANGE( c, p ) ( c ).begin(), ( c ).begin() + ( p ), ( c ).end()

#define SZ( v ) ( (int)( v ).size() )
#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define EXIST( c, e ) ( ( c ).find( e ) != ( c ).end() )

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

// 最長共通部分列を求める
// O( NM )
template < typename RandomAccessIterator, typename OutputIterator >
OutputIterator longest_common_subsequence(
		RandomAccessIterator first1, RandomAccessIterator last1, 
		RandomAccessIterator first2, RandomAccessIterator last2,
		OutputIterator result )
{
	using T = typename std::iterator_traits< RandomAccessIterator >::value_type;
	const int N = std::distance( first1, last1 ), M = std::distance( first2, last2 );

	std::vector< std::vector<int> > dp( N + 1, std::vector<int>( M + 1, 0 ) );
	std::vector< std::vector< std::pair<int,int> > > prev( N + 1, std::vector< std::pair<int,int> >( M + 1, std::make_pair( -1, -1 ) ) );

	for ( int i = 0; i < N; ++i )
	{
		for ( int j = 0; j < M; ++j )
		{
			if ( *( first1 + i ) == *( first2 + j ) )
			{
				dp[ i + 1 ][ j + 1 ] = dp[i][j] + 1;
				prev[ i + 1 ][ j + 1 ] = std::make_pair( i, j );
			}
			else
			{
				if ( dp[ i + 1 ][j] < dp[i][ j + 1 ] )
				{
					dp[ i + 1 ][ j + 1 ] = dp[i][ j + 1 ];
					prev[ i + 1 ][ j + 1 ] = std::make_pair( i, j + 1 );
				}
				else
				{
					dp[ i + 1 ][ j + 1 ] = dp[ i + 1 ][j];
					prev[ i + 1 ][ j + 1 ] = std::make_pair( i + 1, j );
				}
			}
		}
	}

	std::vector<T> lcs;
	lcs.reserve( dp[N][M] );
	for ( std::pair<int,int> current( N, M ); prev[ current.first ][ current.second ].first != -1; current = prev[ current.first ][ current.second ] )
	{
		if ( dp[ prev[ current.first ][ current.second ].first ][ prev[ current.first ][ current.second ].second ] < dp[ current.first ][ current.second ] )
		{
			lcs.push_back( *( first1 + prev[ current.first ][ current.second ].first ) );
		}
	}

	std::reverse_copy( lcs.begin(), lcs.end(), result );
	return result;
}

string solve( const string &a, const string &b )
{
	string res;
	longest_common_subsequence( ALL( a ), ALL( b ), BI( res ) );
	return SZ( res ) == SZ( b ) ? a : "";
}

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );

	string S;
	cin >> S;

	const int L = SZ( S );

	if ( L % 2 == 0 && equal( ALL( S ), S.rbegin() ) )
	{
		S.insert( L / 2, "a" );
		cout << S << endl;
		return 0;
	}
	
	REP( i, 2 )
	{
		string a = S.substr( 0, ( L - 1 ) / 2 + i );
		string b = S.substr( ( L - 1 ) / 2 + ( L - 1 ) % 2 + i );
		reverse( ALL( a ) );

		string res = SZ( a ) < SZ( b ) ? solve( b, a ) : solve( a, b );
		if ( res != "" )
		{
			string rev( res );
			reverse( ALL( rev ) );
			if ( L % 2 == 0 )
			{
				rev += S[ L / 2 ];
			}
			rev += res;
			cout << rev << endl;
			return 0;
		}
	}
	cout << "NA" << endl;

	return 0;
}