import java.util.*;

public class Main {

  public static void main(String[] args) throws Exception {
    Scanner scn = new Scanner(System.in);

    int N = scn.nextInt(), M = scn.nextInt();
    int s[] = new int[M], c[] = new int[M];
    for (int i = 0; i < M; i++) {
      s[i] = scn.nextInt();
      c[i] = scn.nextInt();
    }

    int ans = -1;
    int t = (N != 1 ? (int) Math.pow(10, N - 1) : 0);
    for (int i = t; i < t * 10; i++) {
      Boolean flag = true;
      for (int j = 0; j < M; j++) {
        if (N < j) {
          flag = false;
          break;
        }
        String str = String.valueOf(i);
        String C = String.valueOf(c[j]);
        if (!str.substring(s[j] - 1, s[j]).equals(C)) {
          flag = false;
          break;
        }
      }
      if (flag) {
        ans = i;
        break;
      }
    }

    System.out.println(ans);
  }
}
