import java.util.*;

public class Main {
	public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int M = sc.nextInt();
    int[] dataA = new int[N+1];
    int[] dataB = new int[N+1];
    boolean t = true;
    for(int i=0;i<M;i++){
      int s = sc.nextInt();
      int c = sc.nextInt();
      if(N>1&&s==1&& c==0) {
    	  t = false;
      }
      if(dataB[s]==0){
        dataA[s]=c;
        dataB[s]++;
      }else{
        if(dataA[s] ==c){
          dataB[s]++;
        }else{
          t = false;
          break;
        }
      }
    }
   if(t ==false){
     System.out.print("-1");
     
   }else if(N==1 && dataB[1]==0){
	   System.out.print("0");		 
   }else{
     for(int i =1 ;i<N+1;i++){
    	  if(dataB[i] ==0 && i!=1 &&N>1){
    		       dataA[i]=0;
    		  }else if(dataB[i] ==0 && i==1 && N>1) {
    			  dataA[i]=1;
       }
       System.out.print(dataA[i]);
     }
   }
    
  }
}
