/* -*- coding: utf-8 -*-
 *
 * 2042.cc:  No.2042 RGB Caps - yukicoder
 */

#include<cstdio>
#include<algorithm>
#include<utility>
 
using namespace std;

/* constant */

const int MAX_N = 100000;
const char ccs[] = "RGB";

/* typedef */

typedef pair<int,int> pii;

/* global variables */

int cmap[128], cs[3];
pii acs[MAX_N];
char s[MAX_N + 4];

/* subroutines */

/* main */

int main() {
  for (int i = 0; ccs[i]; i++) cmap[ccs[i]] = i;

  int n, k;
  scanf("%d%d", &n, &k);

  for (int i = 0; i < k; i++) {
    int ai;
    char cis[4];
    scanf("%d%s", &ai, cis);
    acs[i] = pii(ai, cmap[cis[0]]);
  }
  sort(acs, acs + k);

  fill(cs, cs + 3, 0);
  int l = 0;
  for (int i = 0; i < k; i++) {
    int ai = acs[i].first, ci = acs[i].second;
    int mi = (ai + 2) / 3;
    while (l < ai && cs[ci] < mi) s[l++] = ccs[ci], cs[ci]++;

    if (cs[ci] < cs[(ci + 1) % 3] || cs[ci] < cs[(ci + 2) % 3]) {
      puts("-1"); return 0;
    }
  }

  while (l < n) s[l++] = 'R';
  puts(s);
  return 0;
}