import sys
from collections import deque

def main():
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    M = int(input[idx])
    idx += 1
    edges = []
    adj = [[] for _ in range(N+1)]  # 1-based
    for _ in range(M):
        a = int(input[idx])
        idx += 1
        b = int(input[idx])
        idx += 1
        edges.append((a, b))
        adj[a].append(b)
        adj[b].append(a)  # for undirected bipartition check

    # Check if the underlying undirected graph is bipartite
    color = [-1] * (N + 1)
    is_bipartite = True
    for u in range(1, N+1):
        if color[u] == -1:
            queue = deque()
            queue.append(u)
            color[u] = 0
            while queue:
                v = queue.popleft()
                for nei in adj[v]:
                    if color[nei] == -1:
                        color[nei] = color[v] ^ 1
                        queue.append(nei)
                    elif color[nei] == color[v]:
                        is_bipartite = False
                        break
                if not is_bipartite:
                    break
            if not is_bipartite:
                break
    if is_bipartite:
        print(-1)
        return

    # If not bipartite, output alternating R and G
    res = []
    for i in range(M):
        res.append('R' if i % 2 == 0 else 'G')
    print(''.join(res))

if __name__ == '__main__':
    main()