#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
void*wmem;
char memarr[96000000];
template<class T> inline void walloc1d(T **arr, int x, void **mem = &wmem){
  static int skip[16] = {0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
  (*mem) = (void*)( ((char*)(*mem)) + skip[((unsigned long long)(*mem)) & 15] );
  (*arr)=(T*)(*mem);
  (*mem)=((*arr)+x);
}
template<class T> inline void walloc1d(T **arr, int x1, int x2, void **mem = &wmem){
  walloc1d(arr, x2-x1, mem);
  (*arr) -= x1;
}
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
inline void wt_L(const char c[]){
  int i=0;
  for(i=0;c[i]!='\0';i++){
    my_putchar_unlocked(c[i]);
  }
}
struct graph{
  int N;
  int*es;
  int**edge;
  void setDirectEdge(int N__, int M, int A[], int B[], void **mem = &wmem){
    int i;
    N = N__;
    walloc1d(&es, N, mem);
    walloc1d(&edge, N, mem);
    walloc1d(&edge[0], M, mem);
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      es[A[i]]++;
    }
    for(i=(0);i<(N);i++){
      walloc1d(&edge[i], es[i], mem);
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      edge[A[i]][es[A[i]]++] = B[i];
    }
  }
  void getDist(int root, int res[], void *mem = wmem){
    int i;
    int j;
    int k;
    int*q;
    int s;
    int z;
    walloc1d(&q, N, &mem);
    for(i=(0);i<(N);i++){
      res[i]=-1;
    }
    res[root]=0;
    s=0;
    z=1;
    q[0]=root;
    while(z){
      i=q[s++];
      z--;
      for(j=(0);j<(es[i]);j++){
        k=edge[i][j];
        if(res[k]>=0){
          continue;
        }
        res[k]=res[i]+1;
        q[s+z++]=k;
      }
    }
  }
  int getDist(int a, int b, void *mem = wmem){
    int i;
    int j;
    int k;
    int*q;
    int s;
    int z;
    int*d;
    if(a==b){
      return 0;
    }
    walloc1d(&d, N, &mem);
    walloc1d(&q, N, &mem);
    for(i=(0);i<(N);i++){
      d[i] = -1;
    }
    d[a] = 0;
    s = 0;
    z = 1;
    q[0] = a;
    while(z){
      i = q[s++];
      z--;
      for(j=(0);j<(es[i]);j++){
        k = edge[i][j];
        if(d[k] >= 0){
          continue;
        }
        d[k] = d[i] + 1;
        if(k==b){
          return d[k];
        }
        q[s+z++] = k;
      }
    }
    return -1;
  }
  int shortestPath(const int s, const int t, int res[], void *mem=wmem){
    int i;
    int j;
    int k;
    int*q;
    int qs = 0;
    int qe = 0;
    int*b;
    walloc1d(&b, N, &mem);
    walloc1d(&q, N, &mem);
    for(i=(0);i<(N);i++){
      b[i] = -1;
    }
    b[s] = -2;
    q[qe++] = s;
    while(qe > qs){
      i = q[qs++];
      for(j=(0);j<(es[i]);j++){
        k = edge[i][j];
        if(b[k]!=-1){
          continue;
        }
        b[k] = i;
        q[qe++] = k;
      }
      if(b[t]!=-1){
        break;
      }
    }
    if(b[t]==-1){
      return -1;
    }
    k = 0;
    res[k] = i = t;
    while(i != s){
      res[++k] = (i = b[i]);
    }
    std::reverse(res, res+k+1);
    return k;
  }
}
;
int N;
int M;
int A[2000];
int B[2000];
graph g;
int dist[1000][1000];
int ps;
int p[1003];
char res[2002];
int main(){
  int i;
  wmem = memarr;
  int x;
  int y;
  rd(N);
  rd(M);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(M);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);A[Lj4PdHRW] += (-1);
      rd(B[Lj4PdHRW]);B[Lj4PdHRW] += (-1);
    }
  }
  if(M==N){
    wt_L(-1);
    wt_L('\n');
    return 0;
  }
  g.setDirectEdge(N,M,A,B);
  for(i=(0);i<(N);i++){
    g.getDist(i,dist[i]);
  }
  for(i=(0);i<(N);i++){
    int j;
    for(j=(0);j<(N);j++){
      if(dist[i][j]==-1){
        wt_L(-1);
        wt_L('\n');
        return 0;
      }
    }
  }
  for(i=(0);i<(M);i++){
    res[i] = 'R';
  }
  for(i=(0);i<(M);i++){
    if(dist[B[i]][A[i]] % 2 == 0){
      ps = g.shortestPath(B[i], A[i], p);
      p[++ps] = B[i];
      for(i=(0);i<(ps);i+=(2)){
        int j;
        auto V9aVTaxx = ((p[i]));
        auto APIVbQlN = (( p[i+1]));
        x=V9aVTaxx;
        y=APIVbQlN;
        for(j=(0);j<(M);j++){
          if(A[j]==x && B[j]==y){
            res[j] = 'G';
          }
        }
      }
      wt_L(res);
      wt_L('\n');
      return 0;
    }
  }
  wt_L("-1");
  wt_L('\n');
  return 0;
}
// cLay version 20210227-1

// --- original code ---
// int N, M, A[2000], B[];
// graph g;
// int dist[1000][1000], ps, p[1003];
// char res[2002];
// {
//   int x, y;
//   rd(N,M,(A--,B--)(M));
//   if(M==N) wt(-1), return 0;
//   g.setDirectEdge(N,M,A,B);
//   rep(i,N) g.getDist(i,dist[i]);
//   rep(i,N) rep(j,N) if(dist[i][j]==-1) wt(-1), return 0;
//   rep(i,M) res[i] = 'R';
//   rep(i,M) if(dist[B[i]][A[i]] % 2 == 0){
//     ps = g.shortestPath(B[i], A[i], p);
//     p[++ps] = B[i];
//     rep(i,0,ps,2){
//       (x, y) = (p[i], p[i+1]);
//       rep(j,M) if(A[j]==x && B[j]==y) res[j] = 'G';
//     }
//     wt(res), return 0;
//   }
//   wt("-1");
// }