import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N, K;
int[] A;
char[] C;

bool solve() {
  auto cs = new char[N + 1];
  cs[] = '?';
  foreach (k; 0 .. K) {
    if (cs[A[k]] == '?') {
      cs[A[k]] = C[k];
    }
    if (cs[A[k]] != C[k]) {
      return false;
    }
  }
  
  debug {
    writeln("cs = ", cs);
  }
  foreach (i; 0 .. N + 1) {
    if (cs[i] == '?') {
      cs[i] = 'R';
    }
  }
  auto ans = new char[N];
  foreach (i; 0 .. N) {
    final switch (i % 3) {
      case 0: {
        ans[i] = cs[i + 1];
      } break;
      case 1: {
        if (ans[i - 1] == cs[i + 1]) {
          ans[i] = (ans[i - 1] == 'R') ? 'G' : 'R';
        } else {
          ans[i] = cs[i + 1];
        }
      } break;
      case 2: {
        if (ans[i - 2] == cs[i + 1] || ans[i - 1] == cs[i + 1]) {
          ans[i] = 'R' ^ 'G' ^ 'B' ^ ans[i - 2] ^ ans[i - 1];
        } else {
          ans[i] = cs[i + 1];
        }
      } break;
    }
  }
  writeln(ans);
  return true;
}

void main() {
  try {
    for (; ; ) {
      N = readInt;
      K = readInt;
      A = new int[K];
      C = new char[K];
      foreach (k; 0 .. K) {
        A[k] = readInt;
        C[k] = readToken[0];
      }
      
      const res = solve;
      if (!res) {
        writeln("-1");
      }
    }
  } catch (EOFException e) {
  }
}