#include <bits/stdc++.h>
using namespace std;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
template <typename T>
inline istream& operator>>(istream& i, vector<T>& v) {
  for (int j = 0; j < ((int)(v).size()); ++j) i >> v[j];
  return i;
}
template <typename T>
string join(const vector<T>& v) {
  stringstream s;
  for (int i = 0; i < ((int)(v).size()); ++i) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T>
inline ostream& operator<<(ostream& o, const vector<T>& v) {
  if ((int)(v).size()) o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream& operator>>(istream& i, pair<T1, T2>& v) {
  return i >> v.first >> v.second;
}
template <typename T1, typename T2>
inline ostream& operator<<(ostream& o, const pair<T1, T2>& v) {
  return o << v.first << "," << v.second;
}
template <typename T>
inline long long int suma(const vector<T>& a) {
  long long int res(0);
  for (auto&& x : a) res += x;
  return res;
}
const double eps = 1e-10;
const long long int LINF = 1001002003004005006ll;
const int INF = 1001001001;
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};
int main() {
  int n, m;
  cin >> n >> m;
  vector<int> a(n, -1);
  for (int i = 0; i < (m); ++i) {
    int s, c;
    cin >> s >> c;
    s--;
    if (a[s] != -1 && a[s] != c) {
      cout << -1 << endl;
      return 0;
    }
    a[s] = c;
  }
  int s = 0;
  for (int i = 0; i < (n); ++i) {
    if (a[i] != -1) {
      s += a[i] * pow(10, n - i - 1);
    }
  }
  int t = s, d = 0;
  while (t) {
    t /= 10;
    d++;
  }
  if (d != n)
    cout << -1 << endl;
  else
    cout << s << endl;
  return 0;
}
