#include <bits/stdc++.h>
#include <bits/extc++.h>

using namespace std;
#define _rep(n, a, b) for (ll n = (a); n <= (b); ++n)
#define _rev(n, a, b) for (ll n = (a); n >= (b); --n)
#define _for(n, a, b) for (ll n = (a); n < (b); ++n)
#define _rof(n, a, b) for (ll n = (a); n > (b); --n)
#define oo 0x3f3f3f3f
#define ll long long
#define db double
#define eps 1e-8
#define bin(x) cout << bitset<10>(x) << endl;
#define what_is(x) cerr << #x << " is " << x << endl
#define met(a, b) memset(a, b, sizeof(a))
#define all(x) x.begin(), x.end()
#define pii pair<ll, ll>
#define pdd pair<db, db>
#define pi acos(-1.0)
const ll maxn = 2e5 + 100;
const ll mod = 1e9;
int c[10];
int val[10];
map<int, int> mat;
signed main(){
	int n, m;
	cin >> n >> m;
	bool okay	= 1;
	_rep(i, 1, m){
		int s, c;
		cin >> s >> c;
		if(mat[s]==c || !mat.count(s)){
			mat[s] = c;
		}else okay = 0;
	}
	if(okay){
		string s;
		_rep(i, 1, n){
			if(mat[i]){
				s+= mat[i] + '0';
			}else s+='0';
		}
		if(s[0] == '0')cout << -1 << endl;
		else cout << s << endl;
	}else cout << -1 << endl;

}