#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <cassert>
#include <numeric>
#include <functional>
#include <ctime>
#pragma warning(disable:4996) 

//#define ATCODER
#ifdef ATCODER
#include <atcoder/all>
#endif

typedef long long ll;
typedef unsigned long long ull;
#define LINF  9223300000000000000
#define LINF2 1223300000000000000
#define LINF3 1000000000000
#define INF 2140000000
const long long MOD = 1000000007;
//const long long MOD = 998244353;

using namespace std;
#ifdef ATCODER
using namespace atcoder;
#endif


void solve()
{
    int n, m;
    scanf("%d%d", &n, &m);
    vector<vector<pair<int,int>>> g(n);   // to,edge
    vector<int> a(m), b(m);
    int i;
    for (i = 0; i < m; i++) {
        scanf("%d%d", &a[i], &b[i]); a[i]--; b[i]--;
        g[a[i]].push_back(make_pair(b[i],i));
    }
    for (i = 0; i < m; i++) {
        int ans0 = 0;
        vector<int> pr(n);
        vector<int> dist(n, INF);
        queue<int> que;
        que.push(b[i]); dist[b[i]] = 0;
        while (!que.empty()) {
            int tmp = que.front(); que.pop();
            int k;
            for (k = 0; k < (int)g[tmp].size(); k++) {
                int ne = g[tmp][k].first;
                if (dist[ne] == INF) {
                    dist[ne] = dist[tmp] + 1;
                    pr[ne] = g[tmp][k].second;
                    que.push(ne);
                }
            }
        }
        int j;
        for (j = 0; j < n; j++) {
            if (dist[j] == INF) {
                printf("-1");
                return;
            }
        }
    }
    for (i = 0; i < m; i++) {
        vector<int> pr(n);
        vector<int> dist(n, INF);
        queue<int> que;
        que.push(b[i]); dist[b[i]] = 0;
        while (!que.empty()) {
            int tmp = que.front(); que.pop();
            int k;
            for (k = 0; k < (int)g[tmp].size(); k++) {
                int ne = g[tmp][k].first;
                if (dist[ne] == INF) {
                    dist[ne] = dist[tmp] + 1;
                    pr[ne] = g[tmp][k].second;
                    que.push(ne);
                }
            }
        }
        if (dist[a[i]] % 2 == 0) {
            vector<int> ans(m);
            int curr = a[i];
            int cnt = 0;
            while (1) {
                int edge = pr[curr];
                ans[edge] = 1 - cnt; cnt = 1 - cnt;
                curr = a[edge];
                if (curr == a[i]) break;
            }
            int k;
            for (k = 0; k < m; k++) {
                if (ans[k]) printf("R");
                else printf("G");
            }
            printf("\n");
            return;
        }
    }
    printf("-1\n");
    return;
}


int main()
{
#if 1
    solve();
#else    
    int T, t;
    scanf("%d", &T);
    for (t = 0; t < T; t++) {
        //cout << "Case #" << t + 1 << ": ";
        solve();
    }
#endif
    return 0;
}