#include<bits/stdc++.h>
using namespace std;
int main(){int n,m,i,s,c,ans,k;
cin>>n>>m;
int a[n];
for(i=0;i<n;i++){a[i]=-1;}
for(i=0;i<m;i++){cin>>s>>c;
s--;
    if(a[s]!=-1&&a[s]!=c){cout<<"-1";return 0;}
    a[s]=c;
}
if(n==1){cout<<max(0,a[0]);return 0;}
if(n==1&&m==0){cout<<"0";return 0;}
if(!a[0]){cout<<"-1";return 0;}
if(a[0]==-1){a[0]=1;}
k=0;
for(i=0;i<n;i++){a[i]=max(k,a[i]);}
for(i=0;i<n;i++){cout<<a[i];}
    return 0;}