#include <iostream>
#include <map>
#include <vector>
#include <queue>
#include <functional>

using namespace std;

typedef vector<string> vs;
typedef tuple<int, string, char> path;

map<string, map<string, vs> > G;
map<string, int> D;
string s, e;
int n, m;

string dijkstra()
{
    D.clear();
    priority_queue<path, vector<path>, greater<path> > PQ;
    PQ.emplace(0, s, '0');
    D[s] = 0;
    while (!PQ.empty())
    {
        string u;
        char c;
        int d;
        tie(d, u, c) = PQ.top(); PQ.pop();

        if (u == e)
            return to_string(d);

        if (D[s] && D[s] < d)
            continue;

        for (auto it : G[u])
        {
            string v = it.first;
            for (string a : it.second)
                if (D.find(v) == D.end() || (a[0] != c && D[v] > D[u] + a.size()))
                    D[v] = d + a.size(), PQ.emplace(d + a.size(), v, a[0]);
        }
    }

    return "impossivel";
}

int main()
{
    while (cin >> m, m)
    {
        string a, b, d;
        cin >> s >> e;
        for (int i = 0; i < m; ++i)
        {
            cin >> a >> b >> d;
            G[a][b].push_back(d);
            G[b][a].push_back(d);
        }

        cout << dijkstra() << '\n';
    }
}