#include <cstdio>
#include <cstring>
#include <algorithm>
#include <stack>
#include <vector>
#include <string>
#include <cassert>
using namespace std;

char word[2][110];
char c[100010];
int K, N, X, Y;
int diff[110];

vector<vector<int> > found[2];

pair<int,int> KMP(const string& str, const string& wzo) {
	#define KMPH(z) while(k>0 && wzo[k] != z[q]) k = p[k]; \
	if (wzo[k] == z[q]) k++; 
	
	int p[wzo.size()+1], k = 0, q, m;
	p[1] = 0;
	
	for(q=1; q<wzo.size(); q++) {
		KMPH(wzo);
		p[q+1] = k;
	}
	
	m = q;
	k = 0;
	
	int A = 23456789;
	int B = -1;
	
	for(q=0; q<str.size(); q++) {
		KMPH(str);
		if (m == k) {
			A = min(A, q - m + 1);
			B = max(B, q - m + 1);
			k = p[k];
		}
	}
	
	return make_pair(A, B);
}

int mod26(int a) {
	assert(a < 100);
	while(a >= 26) a-= 26;
	return a;
}

void wynajdz(int dlugosc, int a) {
	int m = strlen(word[a]);
	for(int i=0; i+m <= N; i++) {
		for(int j=0; j<dlugosc; j++) {
			diff[(i+j) % dlugosc] = (52 + c[i+j] - word[a][j] + 1) % 26; 
		}
		bool ok = true;
		for(int j=dlugosc; j<m; j++) {
			if (diff[(i+j) % dlugosc] != (52 + c[i+j] - word[a][j] + 1) % 26) {
				ok = false;
				break;
			} 
		}
		
		if (ok) {
			vector<int> tmp(diff, diff+dlugosc);
			found[a].push_back(tmp);
		}
	}
}

string wynik, gen;
bool cokolwiek;

void generate(const vector<int>& cykl) {
	gen = "";
	int pos = 0;
	for(int i=0; i<N; i++) {
		gen += char(mod26(c[i] - 'A' + 26 - cykl[pos] + 1) + 'A'); 
		pos++;
		if (pos == cykl.size()) pos = 0;
	}
}

bool check() {
	if (cokolwiek) {
		if (gen != wynik){
			printf("ambiguous\n");
			return false;
		}	
	} else {
		cokolwiek = true;
		wynik = gen;
	}
	return true;
}

string wzo0, wzo1;

bool cross(int a, int b) {
	if (a <= b) {
		return a + X > b; 
	} else {
		return b + Y > a;
	}
}

bool not_verify(const vector<int>& cykl) {
	generate(cykl);
	pair<int, int> A = KMP(gen, wzo0);
	pair<int, int> B = KMP(gen, wzo1);
	//printf("%d %d\n", A.first, A.second);
	return cross(A.first, B.second) && cross(A.second, B.first);
	
} 

int main() {
	while(true) {
		cokolwiek = false;
		scanf("%d",&K);
		if (K == 0) return 0;
		
		scanf("%s %s %s", word[0], word[1], c);
		wzo0 = word[0];
		wzo1 = word[1];
		X = strlen(word[0]);
		Y = strlen(word[1]);
		
		N = strlen(c);
		for(int dl = 1; dl <= K; dl++) {
			found[0].clear(); found[1].clear();
			wynajdz(dl, 0); wynajdz(dl, 1);
			
			sort(found[0].begin(), found[0].end());
			sort(found[1].begin(), found[1].end());
			
			int i = 0, j = 0;
			while(i < found[0].size() && j < found[1].size()) {
				if (found[0][i] == found[1][j]) {
					if (not_verify(found[0][i])) {
						if (rand() % 2 == 0) i++;
						else j++;
					} else { 
						if (!check()) {
							goto end;
						}
						i++;
						j++;
					}
				} else if (found[0][i] < found[1][j]) {
					i++;
				} else {
					j++;
				}
			}
		}
		
		if (cokolwiek) printf("%s\n", wynik.c_str());
		else printf("impossible\n");
		end:;
	}
	return 0;
}