#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return a > val ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
const int maxn = 100010;
int n, m;
vector<pair<pair<int, int>, int> > edges;
vector<int> adj[maxn];
int num[maxn];
int x, y, z;
vector<int> vx, vy;
int f[maxn];
int dj[maxn];
int sz[maxn];
void init() {
  for (int i = (0); i < (n); i++) dj[i] = i, sz[i] = 1;
}
int find(int u) { return dj[u] == u ? dj[u] : dj[u] = find(dj[u]); }
void join(int u, int v) {
  int p = find(u);
  int q = find(v);
  if (p != q) {
    dj[p] = q;
    sz[q] += sz[p];
  }
}
void dfs(int u, int k) {
  num[u] = k;
  if (k == 1) {
    x += sz[u];
    vx.push_back(u);
  } else {
    y += sz[u];
    vy.push_back(u);
  }
  for (int i = (0); i < (int((adj[u]).size())); i++) {
    int v = adj[u][i];
    if (!num[v]) {
      dfs(v, k == 1 ? 2 : 1);
    } else if (num[v] == k) {
      z = 1;
    }
  }
}
vector<int> work(int c) {
  vector<int> res;
  init();
  for (int i = (0); i < (n); i++) adj[i].clear(), num[i] = f[i] = 0;
  for (int i = (0); i < (int((edges).size())); i++) {
    int u = edges[i].first.first;
    int v = edges[i].first.second;
    int w = edges[i].second;
    if (c != w) {
      join(u, v);
    }
  }
  for (int i = (0); i < (int((edges).size())); i++) {
    int u = edges[i].first.first;
    int v = edges[i].first.second;
    int w = edges[i].second;
    if (c == w) {
      int p = find(u);
      int q = find(v);
      if (p == q) {
        res.push_back(-1);
        return res;
      }
      adj[p].push_back(q);
      adj[q].push_back(p);
    }
  }
  for (int i = (0); i < (n); i++)
    if (!num[i] && dj[i] == i) {
      x = y = z = 0;
      vx.clear();
      vy.clear();
      dfs(i, 1);
      if (z) {
        res.push_back(-1);
        return res;
      }
      if (x < y) {
        for (int j = (0); j < (int((vx).size())); j++) {
          f[vx[j]] = 1;
        }
      } else {
        for (int j = (0); j < (int((vy).size())); j++) {
          f[vy[j]] = 1;
        }
      }
    }
  for (int i = (0); i < (n); i++) {
    int p = find(i);
    if (f[p]) {
      res.push_back(i);
    }
  }
  return res;
}
int isbad(vector<int> r) { return int((r).size()) && r[0] == -1; }
void solve() {
  cin >> n >> m;
  for (int i = (0); i < (m); i++) {
    int u, v;
    string s;
    cin >> u >> v >> s;
    u--;
    v--;
    int c = s == "B";
    edges.push_back(make_pair(make_pair(u, v), c));
  }
  vector<int> r1 = work(0);
  vector<int> r2 = work(1);
  if (isbad(r1) && isbad(r2)) {
    cout << "-1\n";
    return;
  }
  vector<int> res;
  if (isbad(r1))
    res = r2;
  else if (isbad(r2))
    res = r1;
  else if (int((r1).size()) < int((r2).size()))
    res = r1;
  else
    res = r2;
  cout << int((res).size()) << "\n";
  for (int i = (0); i < (int((res).size())); i++) cout << res[i] + 1 << " ";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  solve();
  return 0;
}
