import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int n = Integer.parseInt(sa[0]);
		int m = Integer.parseInt(sa[1]);
		int[] u = new int[m];
		int[] v = new int[m];
		int[] deg = new int[n];
		for (int i = 0; i < m; i++) {
			sa = br.readLine().split(" ");
			u[i] = Integer.parseInt(sa[0]) - 1;
			v[i] = Integer.parseInt(sa[1]) - 1;
			deg[u[i]]++;
			deg[v[i]]++;
		}
		br.close();

		if (m % 2 == 1) {
			System.out.println(-1);
			return;
		}

		int m2 = m / 2;
		if (deg[0] > m2 || deg[n - 1] > m2) {
			System.out.println(-1);
			return;
		}

		for (int i = 0; i < m; i++) {
			if (u[i] == 0 && v[i] == n - 1) {
				System.out.println(-1);
				return;
			}
		}

		char[] s = new char[m];
		int b = 0;
		for (int i = 0; i < m; i++) {
			if (u[i] == 0) {
				s[i] = 'B';
				b++;
			}
			if (v[i] == n - 1) {
				s[i] = 'R';
			}
		}

		for (int i = 0; i < m; i++) {
			if (s[i] == '\u0000') {
				if (b < m2) {
					s[i] = 'B';
					b++;
				} else {
					s[i] = 'R';
				}
			}
		}
		System.out.println(s);
	}
}