#include <bits/stdc++.h>
using namespace std;

int main(){
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  vector < set<int> > a(n);
 int d, e;
for (int i = 0; i < m; ++i){
	cin >> d >> e;
	a[d-1].insert(e);
}
if (n == 1 && m == 0){
	cout << 0 << "\n"
	return 0;
}
vector<int> b;
for (int i = 0; i < n; ++i){
	if (i==0){
		if (a[i].empty()){
			b.push_back(1);
		}else if (a[i].size() > 1){
			cout << -1 << "\n";
			return 0;
		}else{
			if(*a[i].begin() == 0){
				cout << -1 << "\n";
				return 0;
			}else{
			b.push_back(*a[i].begin());
			}
		}
	}else{
		if (a[i].size() > 1){
			cout << -1 << "\n";
			return 0;
		}else if (a[i].empty()){
			b.push_back(0);
		}else{
			b.push_back(*a[i].begin());
		}
	}
}
for (int i : b){
	cout << i;
}
cout << "\n";
return 0;
}
