#include<iostream>
#include<vector>
using namespace std;
int main() {
  int N,M;
  cin >> N >> M;
  vector<int> A(N,-1);
  if(N != 1)A[0] = 0;
  int i,s,c;
  bool F = true;
  for(i=0;i<M;i++) {
    cin >> s >> c;
    if(A[s-1] < 0)A[s-1] = c;
    else F = false;
  }
  if(!F)cout << -1 << endl;
  else {
    for(i=0;i<N;i++)cout << max(A[i],0);
    cout << endl;
  }