#include <bits/stdc++.h> using namespace std; #include <atcoder/all> using namespace atcoder; using mint = atcoder::static_modint<998244353>; // using mint = atcoder::static_modint<1000000007>; using ld = long double; using ll = long long; #define mp(a,b) make_pair(a,b) #define rep(i,s,n) for(int i=s; i<(int)n; i++) using vl = vector<ll>; using vvl = vector<vl>; using vvvl = vector<vvl>; using vvvvl = vector<vvvl>; const vector<int> dx{1,0,-1,0},dy{0,1,0,-1}; int main(){ ios::sync_with_stdio(false); cin.tie(nullptr); int n,m;cin >> n >> m; vvl G(n); vl u(m),v(m); rep(i,0,m){ cin >> u[i] >> v[i]; u[i]--,v[i]--; G[u[i]].push_back(v[i]); G[v[i]].push_back(u[i]); } if(m&1){ cout << -1; return 0; } queue<int> Q; vector<bool> used(n); Q.push(0); used[0]=true; while(Q.size()){ auto x=Q.front();Q.pop(); for(auto y:G[x])if(!used[y] && y!=n-1){ used[y]=true; Q.push(y); } } string ans=string(m,'.'); int cntr=0; rep(i,0,m)if(used[u[i]] && v[i]==n-1)ans[i]='R',cntr++; rep(i,0,n)used[i]=false; Q.push(n-1); used[n-1]=true; while(Q.size()){ auto x=Q.front();Q.pop(); for(auto y:G[x])if(!used[y] && y!=0){ used[y]=true; Q.push(y); } } rep(i,0,m)if(u[i]==0 && used[v[i]])ans[i]='B'; rep(i,0,m)if(ans[i]=='.'){ if(cntr*2<m)ans[i]='R',cntr++; else ans[i]='B'; } cout << ans; }