#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(void)
{
  int n,m;
  cin >> n >> m;
  int a[3] = {10,10,10};
  int f = 0;
  for(int i = 0;i < m;i++)
  {
     int b,c;
     cin >> b >> c;
     if(a[b-1] > 9) a[b-1] = c;
     else if(a[b-1] != c)
     {
         f = 1;
         break;
     }
     else a[b-1] = c;
  }
  for(int i = 0;i < 3;i++)
  {
      if(a[i] > 9) a[i] = 0;
  }
  if(f == 1) cout << -1 << endl;
  else if(a[0] == 0 && a[1] == 0]) cout << a[2] << endl;
  else if(a[0] == 0) cout << a[1] << a[2] << endl;
  else cout << a[0] << a[1] << a[2] << endl;
  return 0;
}
