import java.io.*;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  
    String[] line1 = br.readLine().split(" ");
  
    int n = Integer.parseInt(line1[0]);
    int m = Integer.parseInt(line1[1]);
    int[][] data = new int[2][m];
    String[] dataStr;
  
    for (int i = 0; i < m; i++) {
      dataStr = br.readLine().split(" ");
      data[0][i] = n + 1 - Integer.parseInt(dataStr[0]);
      data[1][i] = Integer.parseInt(dataStr[1]);
    }
    if (n == 1) {
      for (int i = 0; i < 10; i++) {
        for (int j = 0; j < m; j++) {
          if (i != data[1][j]) {
            break;
          }
          if (j == m - 1) {
            System.out.println(i);
            return;
          }
        }
      }
    } else {
      for (int i = (int) Math.pow(10, n - 1); i < Math.pow(10, n); i++) {
        for (int j = 0; j < m; j++) {
          if ((i % (int) (Math.pow(10, data[0][j]))) / (int) Math.pow(10, data[0][j] - 1) != data[1][j]) {
            break;
          }
          if (j == m - 1) {
            System.out.println(i);
            return;
          }
        }
      }
    }
    System.out.println(-1);
  }
}
