import java.util.*;

public class Main {
    final static int MOD = 1000000007;
    
    public static void main(String[] args) {
        MyScanner in = new MyScanner();
        int n = in.nextInt();
        int m = in.nextInt();
        int[][] s = in.nextIntMatrix(m,2);
        
        int ans = -1;
        boolean frag = true;
    
        for (int i = 702; i < 1001; i++) {
            String st = String.valueOf(i);
            for (int j = 0; j < m; j++) {
                int a = s[j][1];
                int b = s[j][0]-1;
                char c = st.charAt(s[j][0]-1);
                if (s[j][1] != Character.getNumericValue(st.charAt(s[j][0]-1))) {
                    frag = false;
                    break;
                }
                if (frag) {
                    ans = i;
                }
            }
            
        }
        System.out.println(ans);
        
    }
    
    static class MyScanner {
        static Scanner sc = new Scanner(System.in);
        
        String next() {
            return sc.next();
        }
        
        String nextLine() {
            return sc.nextLine();
        }
        
        int nextInt() {
            return Integer.parseInt(sc.next());
        }
        
        double nextDouble() {
            return Double.parseDouble(next());
        }
        
        long nextLong() {
            return Long.parseLong(next());
        }
        
        int[] nextIntArray(int n) {
            int[] array = new int[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextInt();
            }
            return array;
        }
        
        double[] nextDoubleArray(int n) {
            double[] array = new double[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextDouble();
            }
            return array;
        }
        
        long[] nextLongArray(int n) {
            long[] array = new long[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextLong();
            }
            return array;
        }
        
        String[] nextLineArray(int n) {
            String[] array = new String[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextLine();
            }
            return array;
        }
        
        int[][] nextIntMatrix(int n, int m) {
            int[][] matrix = new int[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    matrix[i][j] = nextInt();
                }
            }
            return matrix;
        }
        
        double[][] nextDoubleMatrix(int n, int m) {
            double[][] matrix = new double[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    matrix[i][j] = nextDouble();
                }
            }
            return matrix;
        }
        
        long[][] nextLongMatrix(int n, int m) {
            long[][] matrix = new long[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    matrix[i][j] = nextLong();
                }
            }
            return matrix;
        }
    }
}