#include <cstdio>
#include <vector>
#include <map>
#include <algorithm>
#include <tr1/unordered_set>
#include <set>
#include <string>
#include <cstring>

using namespace std;
using namespace std::tr1;

char buf[123456];
char cr[2][123];

char sub(char a, char b) {
	int v = (a-'A')-(b-'A')-1;
	while (v < 0) v+=26;
	return 'A'+v;
}

typedef long long ll;
ll mod = 1000000009;
ll q = 65537;

void decrypt(char *b, char *c, int K, set<pair<string, int> >& out) {
	int lb = strlen(b);
	int lc = strlen(c);
	for (int i = 0; i < lb-lc+1; i++) {
		char kk[123];
		for (int j = 0; j < lc; j++) {
			kk[j] = sub(b[i+j], c[j]);
		}
		ll hh[123];
		hh[0] = 0;
		ll moc[123];
		moc[0] = 1;
		for (int j = 0; j < 123; j++) {
			hh[j+1] = (q*hh[j]+kk[j])%mod;
			moc[j+1] = (q*moc[j])%mod;
		}
		bool bad[123];
		for (int j = 0; j < 123; j++) bad[j] = false;
		for (int j = 1; j <= K; j++) {
			if (bad[j]) continue;
			bool ok = true;
			/*ll hash = hh[j];
			for (int x = j; x < lc; x+=j) {
				if (x+j <= lc) {
					ll sub = (hh[x]*moc[j])%mod;
					ll exp = hh[x+j]-sub;
					while (exp < 0) exp+=mod;
					if (exp != hash) {
						ok = false;
						break;
					}	
				} else {
					ll sub = (hh[x]*(moc[lc-x]))%mod;
					ll exp = hh[lc]-sub;
					while (exp < 0) exp+=mod;
					if (exp != hh[lc%j]) {
						ok = false;
						break;
					}
				}
			}*/
			for (int x = j; x < lc; x+=j) {
				if (kk[x]!=kk[x-j]) {
					ok = false;
					break;
				}
			}	
			if (ok) {
				for (int k = j; k < 123; k+=j)
					bad[k] = true;
				char kkk[123];
				int shift = i%j;
				kkk[j] = 0;
				for (int k = 0; k < j; k++) {
					int pp = k-shift;
					while(pp < 0) pp+=j;
					kkk[k] = kk[pp];
				}
				out.insert(make_pair(kkk, i));
			}
		}		
	}
}

void go(int K) {
	scanf("%s %s %s", cr[0], cr[1], buf);	
	set<pair<string, int> > key[2];
	decrypt(buf, cr[0], K,key[0]);
	decrypt(buf, cr[1], K,key[1]);
	unordered_set<string> keys;
	int l[2];
	l[0] = strlen(cr[0]);
	l[1] = strlen(cr[1]);
/*	for (unordered_set<string>::iterator it = key[1].begin(); it != key[1].end(); ++it) {
		printf("%s\n", it->c_str());
	}
	printf("druhe\n");
	for (unordered_set<string>::iterator it = key[0].begin(); it != key[0].end(); ++it) {
		printf("%s\n", it->c_str());
		if (key[1].count(*it)) {
			if (keys.size() == 1) {
				printf("ambiguous\n");
				return;
			}
			keys.insert(*it);
		}
	}*/
	for (set<pair<string, int> >::iterator it = key[0].begin();
		 it != key[0].end(); ++it) {
		pair<string, int> ff = make_pair(it->first, it->second+l[0]);
		set<pair<string, int> >::iterator it2 = key[1].lower_bound(ff);
		if (it2 == key[1].end()) continue;
		if (it2->first == it->first) {
			if (keys.size() == 1) {
				printf("ambiguous\n");
				return;
			}
			keys.insert(it->first);
		}
	}
	for (set<pair<string, int> >::iterator it = key[1].begin();
		 it != key[1].end(); ++it) {
		pair<string, int> ff = make_pair(it->first, it->second+l[1]);
		set<pair<string, int> >::iterator it2 = key[0].lower_bound(ff);
		if (it2 == key[0].end()) continue;
		if (it2->first == it->first) {
			if (keys.size() == 1) {
				printf("ambiguous\n");
				return;
			}
			keys.insert(it->first);
		}
	}
	if (keys.size() == 0) {
		printf("impossible\n");
		return;
	}
	for (int i = 0; buf[i]; i++) {
		string key = *keys.begin();
		printf("%c", sub(buf[i], key[i%(key.length())]));
	}
	printf("\n");
}

int main() {
	while(true) {
		int K; scanf("%d", &K);
		if (K == 0) break;
		go(K);
	}
}