#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <cctype>
#include <string>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <algorithm>
using namespace std;

#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define FOR(i, b) for (i = (0); i < (b); ++i)
#define FOR2(i, a, b) for (int i = (a); i > (b); --i)
inline bool EQ(double a, double b) { return fabs(a-b) < 1e-9; }

const int INF = 1<<29;
typedef long long ll;

int K,A,B,N;
char a[111],b[111],s[111111],ten[111];
int k,tenk,key[111];
multimap<long long,int> se;
multimap<long long,int>::iterator it;

long long hash(){
	long long res=0,Q=10007,i;
	FOR(i,k) res=res*Q+key[i];
	return res;
}

void solve(){
	int i,j,jj,res;
	long long h;
	bool ok;
	char c;
	res=0;
	se.clear();
	for(i=0;i+A<=N;i++){
		for(k=1;k<=K;k++){
			FOR(j,k) key[j]=-1;
			ok=true;
			FOR(j,A){
				c=(-a[j]+s[i+j]+25)%26;
				if(key[(i+j)%k]==-1) key[(i+j)%k]=c;
				else{					
					if(key[(i+j)%k]!=c){
						ok=false;
						break;
					}
				}
			}
			if(!ok) continue;
			se.insert(make_pair(hash(),i));
		}
	}
	for(k=1;k<=K;k++){
		for(i=0;i+B<=N;i++){
			FOR(j,k) key[j]=-1;
			ok=true;
			FOR(j,B){
				c=(-b[j]+s[i+j]+25)%26;
				if(key[(i+j)%k]==-1) key[(i+j)%k]=c;
				else{					
					if(key[(i+j)%k]!=c){
						ok=false;
						break;
					}
				}
			}
			if(!ok) continue;
			h=hash();
			/*tmp=se.count(hash());
			if(res&&tmp){
				bool ok=true;
				FOR(i,k*tenk){ if(key[i%k]+'A'!=ten[i%tenk]){ ok=false; break; } }
				if(ok) continue;
				printf("ambiguous\n");
				return;
			}
			if(tmp&&res==0){
				res=1;
				FOR(j,k) ten[j]=key[j]+'A';
				tenk=k;
				ten[k]='\0';
				//printf("%s\n",ten);
			}*/
			it=se.lower_bound(h);
			while(it!=se.end()&&(*it).first==h){
				j=(*it).second;
				it++;
				//printf("i:%d j:%d\n",i,j);
				if(j+A<=i||i+B<=j){
					if(res){
						bool ok=true;
						FOR(jj,k*tenk){ if(key[jj%k]+'A'!=ten[jj%tenk]){ ok=false; break; } }
						if(ok) continue;
						printf("ambiguous\n");
						return;
					}
					else{
						res=1;
						FOR(jj,k) ten[jj]=key[jj]+'A';
						tenk=k;
						ten[k]='\0';
					}
				}
			}
		}
	}
	if(!res){ printf("impossible\n"); return; }
	int len1 = tenk, len2 = N;
	FOR(i, len2)
	{
		int c = s[i]-'A', d = ten[i%len1]-'A'+1;
 		c = (c-d+26*2) % 26;
		s[i] = c+'A';
	}
	printf("%s\n", s);
}

int main() {
	while(true){
		scanf("%d",&K);
		if(!K) break;
		scanf("%s",a);
		scanf("%s",b);
		scanf("%s",s);
		A=strlen(a);
		B=strlen(b);
		N=strlen(s);
		solve();
	}
	return 0;
}