Babel

#include <bits/stdc++.h>
using namespace std;
#define PB push_back
#define ZERO (1e-10)
#define INF (1<<29)
#define CL(A,I) (memset(A,I,sizeof(A)))
#define DEB printf("DEB!\n");
#define D(X) cout<<"  "<<#X": "<<X<<endl;
#define EQ(A,B) (A+ZERO>B&&A-ZERO<B)
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
#define IN(n) int n;scanf("%d",&n);
#define FOR(i, m, n) for (int i(m); i < n; i++)
#define REP(i, n) FOR(i, 0, n)
#define F(n) REP(i, n)
#define FF(n) REP(j, n)
#define FT(m, n) FOR(k, m, n)
#define aa first
#define bb second
void ga(int N,int *A){F(N)scanf("%d",A+i);}
#define MX (5000)
int ct;
string R[MX];
unordered_map<string,int> t;
int gt(string s){
    auto i(t.find(s));
    if(i==t.end())return R[ct]=s,t[s]=ct++;
    return i->second;
}
#define CLR (ct=0,t.clear())
char s[MX],r[MX],p[MX];
struct EG{int t,v,c;};
vector<EG> g[MX];
int N,f,T,a,b,c;
//DJ
unordered_set<int>cn[MX];
struct eg{
    int v,t,b;
    bool operator<(const eg&r)const{return v>r.v;}
};
int dj(int f,int t){
    F(ct)cn[i].clear();
    priority_queue<eg> q;
    q.push({0,f,0}),cn[f].insert(0);
    while(!q.empty()){
        int a(q.top().t),w(q.top().v),b(q.top().b);
        if(a==t)return w;
        cn[a].insert(b);
        for(auto&h:g[a])if(!(b&1<<h.c))q.push({w+h.v,h.t,1<<h.c});
        while(!q.empty()&&cn[q.top().t].count(q.top().b))q.pop();
    }
    return -1;
}
int main(void){
    while(scanf("%d",&N),N){
        CLR,scanf("%s%s",s,r),f=gt(s),T=gt(r);
        F(N)scanf("%s%s%s",s,r,p),a=gt(s),b=gt(r),g[a].PB({b,c=strlen(p),*p}),g[b].PB({a,c,*p});
        a=dj(f,T);
        if(~a)printf("%d\n",a);
        else printf("impossivel\n");
        F(ct)g[i].clear();
    }
    return 0;
}