#include <algorithm>

#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#include <string>
#include <vector>

using namespace std;

typedef pair< int, int > par;
#define x first
#define y second

#define FORC(it, V) for(__typeof((V).begin()) it = (V).begin(); it != (V).end(); ++it)

const int MAXW = 105;
const int MAXT = 100005;
const int P = 1000000007;

int K, la, lb, lt;
char a[MAXW], b[MAXW], t[MAXT];

int pw[MAXT], KEY[MAXW];
int A[MAXT][MAXW], B[MAXT][MAXW];
par C[MAXT], D[MAXT];

int dd[26][26];

void go( char *a, int l, int A[MAXT][MAXW] ) {
  for( int i = 0; i+l <= lt; ++i ) {
    int h = 0;
    for( int j = 0; j < l; ++j ) {
      A[i][j] = h;
      h = h*P + dd[ t[i+j]-'A' ][ a[j]-'A' ];
    }
    A[i][l] = h;
  }
}

int f( int X, int LX, int Y, int LY ) {
  return ( X - Y*pw[LX-LY] )*pw[LY] + Y;
}

int gen( int K, int l, int A[MAXT][MAXW], par *C ) {
  int c = 0;
  for( int i = 0; i+l <= lt; ++i ) {
    C[i] = par( f( A[i][K], K, A[i][c], c ), i );
    if( c == 0 ) c = K-1; else c--;
  }
  return lt-l+1;
}

int tmp[105];

int provjeri( par *C, int c, char *a, int l, int K ) {
  sort( C, C + c );
  int g = 0;
  for( int i = 0; i < c; ++i ) {
    int p = C[i].y;
    for( int j = 0; j < K; ++j )
      tmp[j] = dd[ t[p+j]-'A' ][ a[j]-'A' ];
    int ok = 1;
    for( int j = K; j < l; ++j )
      if( tmp[j%K] != dd[ t[p+j]-'A' ][ a[j]-'A' ] ) ok = 0;
    if( ok ) C[g++] = C[i];
  }
  return g;
}

int cnt;

void probaj( int A, int B, int K ) {
  if( A <= B && A+la > B ) return;
  if( B <= A && B+lb > A ) return;
  
  if( cnt ) { cnt = -1; return; }
  cnt = K;
  for( int j = 0; j < K; ++j )
    KEY[j] = dd[ t[A+j]-'A' ][ a[j]-'A' ];
}

int main(void)
{
  pw[0] = 1;
  for( int i = 1; i < MAXT; ++i )
    pw[i] = pw[i-1]*P;

  int K;
  for( int i = 0; i < 26; ++i )
    for( int j = 0; j < 26; ++j ) {
      int c = ( i+j+1 )%26;
      dd[c][j] = i;
    }

  for( ;; ) {
    scanf( "%d", &K );
    if( K == 0 ) break;

    scanf( "%s", a ); la = strlen( a );
    scanf( "%s", b ); lb = strlen( b ); 
    scanf( "%s", t ); lt = strlen( t );
    
    go( a, la, A ), go( b, lb, B );

    cnt = 0;
    for( int i = 0; i < K; ++i ) {
      int c = gen( i, la, A, C );
      int d = gen( i, lb, B, D );
      
      c = provjeri( C, c, a, la, i );
      d = provjeri( D, d, b, lb, i );

      int p = 0;
      for( int j = 0; j < c; ++j ) {
	while( p < d && D[p].x < C[j].x ) p++;
	if( p == d || D[p].x != C[j].x ) continue;

	int L = p;
	while( p < d && D[p].x == C[j].x ) p++;
	int R = p-1;

	int l = i, r = i;
	while( r < c && C[r].x == C[j].x ) r++;
	r--;

	probaj( L, l, i ), probaj( L, r, i );
	probaj( R, r, i ), probaj( R, l, i );
	j = r;
      }
    }

    if( cnt == 0 )
      puts( "impossible" );
    else if( cnt == -1 )
      puts( "ambiguous" );
    else {
      for( int i = 0; i < lt; ++i )
	putchar( dd[ t[i]-'A' ][ KEY[i%cnt] ] + 'A' );
    }
  }
  return 0;
}