#include<bits/stdc++.h>
using namespace std;
int main()
{
    int digitnum;
    int opnum;
    cin>>digitnum>>opnum;
    vector<int> digit_arr(digitnum,0);
    while(opnum>0)
    {
        int s;
        int c;
        cin>>s>>c;
        if(digit_arr[s-1]>c)
            digit_arr[s-1]=c;
        opnum--;
    }
    if(digitnum>1)
    {
        if(digit_arr[0]==0)
            cout<<-1<<endl;;
        else
        {
            int sum=0;
            int mul=1;
            for(int i=digit_arr.size()-1;i>0;i--)
            {
                sum+=digit_arr[i]*mul;
                mul*=10;
            }
            cout<<sum<<endl;
        }
    }
    else
        cout<<digit_arr[0]<<endl;
    return 0;
}