using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;

namespace ConsoleTest
{
    static class Program
    {
        class Solver
        {
            ModInt Combination(int m, int n)
            {
                var result = new ModInt(1);
                for (int i = 2; i <= m; ++i)                    
                    result *= new ModInt(i);
                for (int i = 2; i <= n; ++i)
                    result /= new ModInt(i);
                int v = m - n;
                for (int i = 2; i <= v; ++i)
                    result /= new ModInt(i);
                return result;
            }

            public void Solve()
            {
                var nm = Ints();
                var n = nm[0];
                var m = nm[1];
                Tuple<int, char>[] s = new Tuple<int, char>[m];

                for (int i = 0; i < m; ++i)
                {
                    var tmp = Console.ReadLine().Split(' ');
                    s[i] = Tuple.Create(int.Parse(tmp[0]) - 1, tmp[1][0]);
                }
                int max = 10;
                for (int i = 0; i < n - 1; ++i) max *= 10;
                //Console.WriteLine($"max : {max}");
                for(int i = max/10;i<max;++i)
                {
                    string test = i.ToString();
                    if(Check(s, test))
                    {
                        Console.WriteLine(test);
                        return;
                    }
                }
                Console.WriteLine("-1");

            }

            public bool Check(Tuple<int, char>[] s, string test)
            {
                for(int i = 0; i<s.Length; ++i)
                {
                    if (test[s[i].Item1] != s[i].Item2) return false;
                }
                return true;
            }


            public int Int()
            {
                return int.Parse(Console.ReadLine());
            }
            public int[] Ints()
            {
                return Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
            }
            public long[] Longs()
            {
                return Console.ReadLine().Split(' ').Select(s => long.Parse(s)).ToArray();
            }
            public int[][] MultiInts(int num)
            {
                var result = new int[num][];
                for (int i = 0; i < num; ++i)
                {
                    result[i] = Ints();
                }
                return result;
            }
            public T[] MultiItems<T>(int num, Func<string, T> parser)
            {
                var result = new T[num];
                for (int i = 0; i < num; ++i)
                {
                    result[i] = parser(Console.ReadLine());
                }
                return result;
            }
        }
        
  

        static void Main(string[] args)
        {
            new Solver().Solve();
        }
    }
    struct ModInt
    {
        const int MOD = 1000000007;
        long Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ModInt(long data) { if ((Data = data % MOD) < 0) Data += MOD; }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator long(ModInt modInt) => modInt.Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator ModInt(long val) => new ModInt(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, ModInt b)
        {
            long res = a.Data + b.Data;
            return new ModInt() { Data = res >= MOD ? res - MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, long b) => a.Data + b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, ModInt b)
        {
            long res = a.Data - b.Data;
            return new ModInt() { Data = res < 0 ? res + MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, long b) => a.Data - b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator *(ModInt a, ModInt b) => new ModInt() { Data = a.Data * b.Data % MOD };
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator /(ModInt a, ModInt b) => a.Data * GetInverse(b);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override string ToString() => Data.ToString();

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        static long GetInverse(long a)
        {
            long div;
            long p = MOD;
            long x1 = 1, y1 = 0, x2 = 0, y2 = 1;
            while (true)
            {
                if (p == 1) return x2;
                div = a / p;
                x1 -= x2 * div;
                y1 -= y2 * div;
                a %= p;
                if (a == 1) return x1;
                div = p / a;
                x2 -= x1 * div;
                y2 -= y1 * div;
                p %= a;
            }
        }
    }

}
