// Ivan Carvalho

// Solution to https://www.beecrowd.com.br/judge/problems/view/1085

#include <iostream>

#include <queue>

#include <string>

#include <unordered_map>

#define MAXN 4001

#define endl '\n'

#define PB push_back

#define MP make_pair

using namespace std;

typedef pair<char, int> i2;

typedef pair<int, i2> i3;

vector<i3> grafo[MAXN];

int processado[200][MAXN], m, n, possivel;

unordered_map<string, int> mapa;

inline void limpa(int x) {

grafo[x].clear();

for (char caractere = 'a'; caractere <= 'z'; caractere++) {

processado[caractere][x] = 0;

}

}

int main() {

ios_base::sync_with_stdio(0);

cin.tie(0);

cout.tie(0);

while (cin >> m && m) {

n = 2;

possivel = 0;

string destino, origem;

cin >> origem >> destino;

mapa.clear();

mapa[origem] = 1;

mapa[destino] = 2;

limpa(1);

limpa(2);

while (m--) {

string s1, s2, s3;

cin >> s1 >> s2 >> s3;

if (!mapa.count(s1)) {

mapa[s1] = ++n;

limpa(n);

}

if (!mapa.count(s2)) {

mapa[s2] = ++n;

limpa(n);

}

char inicial = s3[0];

int tam = s3.size(), u = mapa[s1], v = mapa[s2];

grafo[u].PB(MP(tam, MP(inicial, v)));

grafo[v].PB(MP(tam, MP(inicial, u)));

}

for (char caractere = 'a'; caractere <= 'z'; caractere++) {

processado[caractere][1] = 1;

}

priority_queue<i3, vector<i3>, greater<i3> > Dijkstra;

for (int i = 0; i < grafo[1].size(); i++) {

Dijkstra.push(grafo[1][i]);

}

while (!Dijkstra.empty()) {

i3 davez = Dijkstra.top();

Dijkstra.pop();

int percorrido = davez.first, v = davez.second.second;

char letra = davez.second.first;

if (v == 2) {

cout << percorrido << endl;

possivel = 1;

break;

}

if (processado[letra][v]) continue;

processado[letra][v] = 1;

for (int i = 0; i < grafo[v].size(); i++) {

i3 agora = grafo[v][i];

int u = agora.second.second, peso = agora.first;

char inicial = agora.second.first;

if (inicial == letra) continue;

Dijkstra.push(MP(percorrido + peso, MP(inicial, u)));

}

}

if (possivel == 0) cout << "impossivel" << endl;

}

return 0;

}