#include<bits/stdc++.h>
#define LL long long
#define N ((int)1e5 + 8)
#define MAX ((int)1e9 + 8)
#define fastio ios_base::sync_with_stdio(false),cin.tie(NULL)

using namespace std;

vector < pair < int , int > > edge[N]; /// edge[i] = list of adjacent nodes of node 'i'

int col[N];

vector < int > com[2];

bool dfs(int n , int c)
{
    if(vis[n] == 1){
        if(c == col[n]) return 1;
        return 0;
    }

    com[c].push_back(n);
    vis[n] = 1;
    col[n] = c;
    bool res = 1;
    for(auto p:edge[n]){
        int a = p.first , w = p.second;
        res &= dfs(a , c^w);
    }
    return res;
}

void Insert(vector < int >& vecA , vector < int >& vecB)
{
    for(int a:vecA) vecB.push_back(a);
}

pair < int , vector < int > > Solve(char finalCol , vector < pair < pair < int , int > , char > >& edges)
{
    Init(n);
    for(auto p:edges){
        int a = p.first.first , b = p.first.second;
        int w = (p.second != finalCol);
        AddEdge(a , b , w);
        AddEdge(b , a , w);
    }

    vector < int > ans;

    for(int i = 1 ; i <= n ; i++){
        if(vis[i]  == 0){
            com[0].clear();
            com[1].clear();
            if(dfs(i , 0) == 0) return {-1,ans};
            if(com[0].size() < com[1].size()) Insert(com[0] , ans);
            else Insert(com[1], ans);
        }
    }
    return {ans.size() , ans};
}


int main()
{
    /// problem link: https://codeforces.com/contest/664/problem/D
    int n , m ;
    cin>>n>>m;
    vector < pair < pair < int , int > , char > > edges;
    while(m--){
        int a , b , w;
        char c;
        cin>>a>>b>>c;
        edges.push_back({{a,b} , c});

    }

    pair < int , vector < int > > ans1 , ans2;
    ans1 = Solve('R' , edges);
    ans2 = Solve('B' , edges);

    if(ans1.first != -1 && ans2.first != -1){
        if(ans1.second.size() < ans2.second.size()) Print(ans1.second);
        else Print(ans2.second);
    }
    else if(ans1.first != -1) Print(ans1.second);
    else if(ans2.first != -1) Print(ans2.second);
    else cout<<"-1\n";
}