#include<bits/stdc++.h>
using namespace std; 
int main() 
{long long n,q,index,insert;cin>>n>>q;
 string s(n,'0');
 while(q--)
 {
   cin>>index>>insert;index--;
   //if(s[index]!='0' and s[index]!=(insert+'0'))return cout<<-1,0;
   if(s[index]!='0')
   s[index]=min(s[index],(insert+'0'));
   else s[index]=(insert+'0');
 }
 if(s[0]==0 and n!=1)return cout<<-1,0;
 return cout<<s,0; 
}
