#include <iostream>
#include <cstdio>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <cmath>
#include <algorithm>
using namespace std;
#define REP(i,a,b) for (int i = (a); i <= (b); ++i)
#define FORI(i,n) REP(i,1,n)
#define FOR(i,n) REP(i,0,int(n)-1)
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define vi vector<int>
#define ll long long
#define SZ(x) ((int)(x.size()))
#define IN(x,y) ((y).find((x)) != (y).end())
#define DBG(v) cerr << #v << " = " << (v) << endl;
#define FOREACH(i,t) for (typeof (t.begin()) i = t.begin(); i != t.end(); i++)
#define fi first
#define se second

int n,l1,l2,fnd,K,q1,q2;
char c[100100];
char w1[111], w2[111], t[111];
string res;
pair<string,int> all1[100100], all2[100100];

void add(string x)
{
	if(fnd == 0)
	{
		//printf("add %s\n", x.c_str());
		res = x;
		fnd++;
		return;
	}
	int R = SZ(res);
	FOR(i,SZ(x)) if(res[i%R] != x[i])
	{
		//printf("add+ %s\n", x.c_str());
		fnd++;
		return;
	}
	if(SZ(x) % R != 0)
	{
		fnd++;
		return;
	}
}

void run(char *w, int l, pair<string,int> *all, int &q, int k)
{
	FOR(i,n-l+1)
	{
		bool ok = 1;
		FOR(j,k) t[j] = '#';
		t[k] = 0;
		FOR(j,l)
		{
			int pos = (i+j) % k;
			char cur = 'A' + w[j] - c[i+j];
			if(cur < 'A') cur += 26;
			if(t[pos] != '#' && t[pos] != cur)
			{
				ok = 0;
				break;
			}
			t[pos] = cur;
		}
		if(!ok) continue;
		//printf("all[%d] = %s\n", q, t);
		all[q++] = mp(t,i);
	}
}

int main ()
{
	while(1)
	{
		fnd = 0;
		q1=q2=0;
		scanf("%d", &K);
		if(!K) return 0;
		scanf("%s%s%s", w1, w2, c);
		for(n=0;c[n];n++) ;
		for(l1=0;w1[l1];l1++) ;
		for(l2=0;w2[l2];l2++) ;
		FORI(k,K)
		{
			run(w1, l1, all1, q1, k);
			run(w2, l2, all2, q2, k);
			sort(all1, all1+q1);
			sort(all2, all2+q2);
			for(int i = 0, j = 0; i < q1 && j < q2; )
			{
				if(all1[i].fi == all2[j].fi)
				{
					//printf("all1 = (%s, %d); all2 = (%s, %d)\n", all1[i].fi.c_str(), all1[i].se, all2[j].fi.c_str(), all2[j].se);
					int i1 = i+1, j1 = j+1;
					while(i1 < q1 && all1[i1].fi == all1[i].fi) i1++;
					while(j1 < q2 && all2[j1].fi == all2[j].fi) j1++;
					//printf("i = %d, j = %d, i1 = %d, j1 = %d\n", i,j,i1,j1);
					if(all1[i].se + l1 <= all2[j1-1].se) add(all1[i].fi);
					else if(all1[i1-1].se >= all2[j].se + l2) add(all1[i].fi);
					i = i1;
					j = j1;
				}
				else if(all1[i].fi < all2[j].fi) i++;
				else j++;
			}
			if(fnd==2) break;
		}
		if(fnd == 2) printf("ambiguous\n");
		else if(fnd == 0) printf("impossible\n");
		else
		{
			FOR(i,n)
			{
				c[i] += res[i%res.size()] - 'A';
				if(c[i] > 'Z') c[i] -= 26;
			}
			printf("%s\n", c);
		}
	}
	return 0;
}