#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define repp(i,m,n) for(int (i)=(m);(i)<(n);(i)++)
#define repm(i,n) for(int (i)=(n-1);(i)>=0;(i)--)
#define INF (1ll<<60)
#define all(x) (x).begin(),(x).end()
typedef long long lint;
const lint MOD =1000000007;
const lint MAX = 10000000;
using Graph =vector<vector<lint>>;
typedef pair<lint,lint> P;

 
lint fac[MAX], finv[MAX], inv[MAX];

void COMinit() 
{
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (lint i = 2; i < MAX; i++)
    {
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

long long COM(lint n, lint k)
{
    if (n < k)
        return 0;
    if (n < 0 || k < 0)
        return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

lint primary(lint num)
{
    if (num < 2) return 0;
    else if (num == 2) return 1;
    else if (num % 2 == 0) return 0;

    double sqrtNum = sqrt(num);
    for (int i = 3; i <= sqrtNum; i += 2)
    {
        if (num % i == 0)
        {
            return 0;
        }
    }

    return 1;
}
   long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
    lint lcm(lint a,lint b){
        return a/__gcd(a,b)*b;
    }
    
    
  int main(){
      lint N,M;
      cin>>N>>M;
      lint a[N];
      fill(a,a+N,-1);
      bool x=true;
      rep(i,M){
          lint s,c;
          cin>>s>>c;
          --s;
          if(a[s]==-1||a[s]==c)a[s]=c;
          else x=false;
      }
      bool t=false;
      bool v=false;
      rep(i,N){
          if(a[i]!=0)v=true;
          if(a[i]==0&&!v){
              cout<<-1;
              return 0;
          }
      }
     if(N==1&&a[0]==-1){
         cout<<0;
         return 0;
     }
      if(x){
          rep(i,N){
              if(i==0&&a[i]==-1)a[i]=1;
              if(a[i]==-1)a[i]=0;
              if(a[i]==0&&t==false)continue;
              else {
                  t=true;
                  cout<<a[i];
              }
             
          }
      }else cout<<-1;
  }
     
  
