#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll, ll, ll>>;
using vb = vector<bool>;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const ll INF = 1e9;
const ll MOD = 1e9 + 7;
const ll LINF = 1e18;
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, m;
  cin >> n >> m;
  string s = string(n, '0');
  vb check(n);
  bool ok = true;
  for (long long i = (long long)(0); i < (long long)(m); ++i) {
    ll idx;
    char c;
    cin >> idx >> c;
    idx--;
    if (check[idx]) {
      if (s[idx] != c) ok = false;
    } else {
      s[idx] = c;
      check[idx] = true;
    }
  }
  if (!ok || s[0] == '0' && s.size() != 1) {
    cout << -1 << endl;
    return 0;
  } else {
    cout << s << endl;
  }
}
