#include // #include // #include using namespace std; // using namespace atcoder; // using bint = boost::multiprecision::cpp_int; using ll = long long; using ull = unsigned long long; using ld = long double; using pii = pair; using pll = pair; using vi = vector; using vvi = vector; using vvvi = vector; using ve = vector>; using vb = vector; using vvb = vector; #define rep(i,s,n) for(ll i = (ll)s;i < (ll)n;i++) #define rrep(i,l,r) for(ll i = r-1;i >= l;i--) #define ALL(x) (x).begin(),(x).end() #define sz(c) ((ll)(c).size()) #define LB(A,x) (int)(lower_bound(A.begin(),A.end(),x)-A.begin()) #define UB(A,x) (int)(upper_bound(A.begin(),A.end(),x)-A.begin()) // #define MOD 1000000007 #define MOD 998244353 templateusing min_priority_queue=priority_queue,greater>; templateostream&operator<<(ostream&os,vector&v){for(int i = 0;i < v.size();i++)os<istream&operator>>(istream&is,vector&v){for(T&in:v)is>>in;return is;} templateostream&operator<<(ostream&os,pair&p){os<istream&operator>>(istream&is,pair&p){is>>p.first>>p.second;return is;} template inline bool chmax(T &a,T b){if(a < b){a = b;return true;}return false;} template inline bool chmin(T &a,T b){if(a > b){a = b;return true;}return false;} ld dist(ld x1,ld y1,ld x2, ld y2){return sqrtl((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));} int main(){ ios_base::sync_with_stdio(0), cin.tie(0); int n,m;cin >> n >> m; vector v(n,{-1,-1}); bool is = !(m & 1); int c0 = 0,c1 = 0; rep(i,0,m){ int a,b;cin >> a >> b; a--;b--; if(a == 0)v[b].first = i,c0++; if(b == n-1)v[a].second = i,c1++; if(a == 0 && b == n-1)is = false; } if(!is){ cout << -1 << endl; return 0; } string res = string(m,'G'); if(c0 <= m/2 && c1 <= m/2){ int cnt0 = m/2,cnt1 = m/2; rep(i,0,n){ if(v[i].first != -1)res[v[i].first] = 'R',cnt0--; if(v[i].second != -1)res[v[i].second] = 'B',cnt1--; } rep(i,0,m){ if(res[i] == 'G'){ if(cnt0)res[i] = 'R',cnt0--; else res[i] = 'B',cnt1--; } } }else{ if(c1 <= m/2){ rep(i,0,n)swap(v[i].first,v[i].second); } c0 = m/2; rep(i,0,n){ if(v[i].first != -1){ res[v[i].first] = 'B';c0--; v[i].first = -1; if(v[i].second != -1){ res[v[i].second] = 'R';c1--; v[i].second = -1; } } } rep(i,0,n){ if(v[i].second != -1 && c0){ res[v[i].second] = 'B';c0--; v[i].second = -1; } } rep(i,0,n){ if(v[i].second != -1){ res[v[i].second] = 'R'; } } rep(i,0,m)if(res[i] == 'G')res[i] = 'R'; } // assert(count(ALL(res),'R') == count(ALL(res),'B')); cout << res << endl; return 0; }