import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum K = 26;

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const M = readInt();
      auto A = new int[M];
      auto B = new int[M];
      auto C = new int[M];
      foreach (i; 0 .. M) {
        A[i] = readInt() - 1;
        B[i] = readInt() - 1;
        C[i] = readToken()[0] - 'a';
      }
      
      auto G = new int[][N];
      foreach (i; 0 .. M) {
        G[A[i]] ~= i;
        G[B[i]] ~= i;
      }
      
      DList!int que;
      auto prevI = new int[][](N, K + 1);
      auto prevK = new int[][](N, K + 1);
      foreach (u; 0 .. N) {
        prevI[u][] = -1;
        prevK[u][] = -1;
      }
      foreach (i; G[0]) {
        const v = A[i] ^ B[i] ^ 0;
        prevI[v][C[i]] = i;
        prevK[v][C[i]] = -2;
        que ~= v;
        que ~= C[i];
      }
      for (; !que.empty; ) {
        const u = que.front; que.removeFront;
        const k = que.front; que.removeFront;
        foreach (i; G[u]) {
          const v = A[i] ^ B[i] ^ u;
          const kk = (k == C[i]) ? k : K;
          if (!~prevI[v][kk]) {
            prevI[v][kk] = i;
            prevK[v][kk] = k;
            que ~= v;
            que ~= kk;
          }
        }
      }
      
      if (~prevI[N - 1][K]) {
        int[] ans;
        for (int v = N - 1, kk = K; kk != -2; ) {
          const i = prevI[v][kk];
          const k = prevK[v][kk];
          ans ~= i;
          v = A[i] ^ B[i] ^ v;
          kk = k;
        }
        ans.reverse;
        bool isPalin = true;
        for (int j = 0, k = cast(int)(ans.length) - 1; j < k; ++j, --k) {
          isPalin = isPalin && (C[ans[j]] == C[ans[k]]);
        }
        if (isPalin) {
          ans ~= ans[$ - 1];
          ans ~= ans[$ - 1];
        }
        writeln(ans.length);
        foreach (i; ans) {
          writeln(i + 1);
        }
      } else {
        writeln(-1);
      }
      debug {
        writeln("========");
      }
    }
  } catch (EOFException e) {
  }
}