#include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <cstring>
using namespace std;
#define FI first
#define SE second
#define X first
#define Y second
#define ST first
#define ND second
#define MP make_pair
#define PB push_back
typedef vector<int> VI;
typedef pair<int,int> PII;
typedef long long LL;
typedef long double LD;
typedef double D;
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORD(i,a,b) for(int i=(a);i>=(b);i--)
#define FORE(a,b) for(VAR(a,(b).begin());a!=(b).end();a++)
#define VAR(a,b) __typeof(b) a=(b)
#define SIZE(a) ((int)(a).size())
#define ALL(x) (x).begin(),(x).end()
#define CLR(x,a) memset(x,a,sizeof(x))

int maxKey;

const int N = 100025;

const LL B = 239;

LL h[N];
int t[N];
int nText;

LL pB[2 * N];

LL getHash(int l, int r) {
    if (r < l) {
        return 0LL;
    }
    return (h[r] - (l > 0 ? h[l - 1] : 0)) * pB[nText - r];
}

void gen(char* text, char* crib, vector<pair<LL, PII> >& res) {
    int nCrib = strlen(crib);
    for (int i = 0; i + nCrib <= nText; ++i) {
        for (int j = 0; j < nCrib; ++j) {
            int c = text[i + j] - 'A';
            c -= int(crib[j] - 'A');
            c = (c % 26 + 26) % 26 + 1;
            t[j] = c;
        }
        for (int j = 0; j < nCrib; ++j) {
            int c = t[j];
            if (j > 0) {
                h[j] = h[j - 1] + c * pB[j];
            } else {
                h[j] = c * pB[j];
            }
        }

        for (int j = 1; j <= maxKey; ++j) {
            if (getHash(0, nCrib - j - 1) == getHash(j, nCrib - 1)) {
                int mv = (j - i % j) % j;
                res.PB(MP((getHash(mv, j - 1) + pB[mv] * getHash(0, mv - 1)) * pB[nText - mv], MP(i, j)));
            }
        }
    }
    sort(ALL(res));
    int last = 0;
    for (int i = 0; i < (int) res.size(); ++i) {
        if (last == 0 || res[i].ST  != res[last - 1].ST || i == (int) res.size() - 1 || res[i + 1].ST != res[i].ST) {
            res[last++] = res[i];
        }
    }
    res.erase(res.begin() + last, res.end());
}

vector<pair<LL, PII> > res1, res2;

char text[N];
char crib1[105], crib2[105];

vector<pair<LL, PII> > ans;

int ciph[105];

void alg() {
    scanf("%s %s %s", crib1, crib2, text);
    nText = strlen(text);
    pB[0] = 1;
    for (int i = 1; i <= 2 * nText + 2; ++i) {
        pB[i] = pB[i - 1] * B;
    } 
    res1.clear();
    res2.clear();
    gen(text, crib1, res1);
    gen(text, crib2, res2);
    ans.clear();
    int nCrib1 = strlen(crib1);
    int nCrib2 = strlen(crib2);
    int jj = 0;
    for (int i = 0; i < (int) res1.size(); ++i) {
        while (jj < (int) res2.size() && res2[jj].ST < res1[i].ST) {
            ++jj;
        }
        {
            if (jj < (int) res2.size() && res2[jj].ST == res1[i].ST && (res1[i].ND.ST + nCrib1 <= res2[jj].ND.ST || res2[jj].ND.ST + nCrib2 <= res1[i].ND.ST)) {
                ans.PB(res1[i]);
            }
        }
        {
            ++jj;
            if (jj < (int) res2.size() && res2[jj].ST == res1[i].ST && (res1[i].ND.ST + nCrib1 <= res2[jj].ND.ST || res2[jj].ND.ST + nCrib2 <= res1[i].ND.ST)) {
                ans.PB(res1[i]);
            }
            --jj;
        }
    }
    if (ans.empty()) {
        printf("impossible\n");
        return;
    }
    int mlen = nText +1;
    LL hmlen;
    int smlen;
    FORE (it, ans) {
        if (it->ND.ND < mlen) {
            mlen = it->ND.ND;
            hmlen = it->ST;
            smlen = it->ND.ST;
        }
    }
    FORE (it, ans) {
        LL cur = 0;
        LL hm = hmlen;

        LL por = it->ST;
        for (int j = 0; j < it->ND.ND; j += mlen) {
            cur += hm;
            hm *= pB[mlen];
            if (j > 0) {
                por *= pB[mlen];
            }
        }
        if (cur != por) {
            printf("ambiguous\n");
            return;
        }
    }
    int mv = smlen % mlen;
    for (int j = 0; j < mlen; ++j) {
        ciph[(mv + j) % mlen] = (int(crib1[j]) - int(text[smlen + j % nCrib1]) + 26) % 26;
    }
    for (int i = 0; i < nText; ++i) {
        int c = text[i] - 'A';
        c = (c + ciph[i % mlen]) % 26;
        putchar(c + 'A');
    }
    putchar('\n');
}

int main() {
    while (scanf("%d", &maxKey) != EOF && maxKey != 0) {
        alg();
    }

    return 0;
}